Martin Bucer was John Calvin's "father in the faith."
The ff. is sourced from Brian Lugioyo's Martin Bucer's Doctrine of Justification: Reformation Theology and Early Modern Irenicism (Oxford Studies in Historical Theology):
By faith alone means that in faith Christians contemplate Christ, and because faith comes with the bestowal of the Spirit of Christ, they become possessed by him so that believers now live in him and he in them. In this sense of mutual inhabitation, Christians are allowed to cooperate with God in salvation, since these works are not their own but the work of Christ in them. This agency is expressed in Bucer primarily in terms of the bestowal of the Holy Spirit,  being clothed in Christ,  participation in Christ,  being in communion with Christ, and so forth.  If a believer “does any good, it results from the fact that he is a creation of God, created for good works, works which God himself prepares, makes and performs, so that he rewards in us gifts which are already his.”  Hence, Bucer follows Augustine’s view that “when God crowns our merits, he crowns nothing but his own gifts.”  Merit is not the result of works but the result of the believer cooperating with the Spirit who works within the believer:
Nevertheless, when God wants us to cooperate with him by good works for our salvation, or rather, even to “work it out” (κατεργάζεσθαι) [Phil. 2:10] and has thus determined to repay us according to our deeds, there is brought about also in its own way our justification; that is, eternal life is assigned to us as a result of works. But this is the case only when through our election and the purpose of God formed before the ages, there is already assigned to us before the foundation of the world this life of God as a result of the grace of God and the merit of Christ [Ephesians 1 and 3]. This life moreover is assigned to us through faith, that is, after we believe in Christ and have in some way become already possessed of him. This of course comes about at that blessed beginning of faith which belongs to the sons of God through the Spirit, who is the pledge of this inheritance. For good works are the fruit of this faith and of the Spirit. 
Works are in a sense a cooperating cause, which Bucer speaks about as a secondary cause elsewhere.
In his conciliatio in regard to Romans 2:11–16, Bucer explains the nature of election, faith, and works as triplex iustificatio, that is, God assigns eternal life to Christians in three ways: 
The first is that by which he destines us to eternal life and it exists solely by his goodness and regard for the merit of Christ . . . . The second way is that by which he already reveals eternal life in some way and grants that one enjoy it, by the gift of his Spirit, in which we cry “Abba Father.” This justification exists in addition by reason of our faith, but that too is something which God out of his free goodness gives, and brings about by his Spirit in us. The third way is when he now reveals actually and fully the eternal life or even blessings which we enjoy, no longer only by faith and hope. Our deeds contribute to this justification, but they too are the gifts and works of the free goodness of God. 
The first iustificatio is equated with election and acts as the foundation for the following two iustificationes, that is, faith and works.  As he states thereafter, “The sole goodness of God and merit of Christ is the prime and in itself entire cause.”  God’s divine benevolence is the primary and sole cause of salvation; his election enables the elect to do good works, which then function only as secondary causes.
Later on, in reference to Romans 4:1–8, Bucer speaks of a prima iustificatio, that is, “absolution from all impiety,”  and a secundaria iustificatio,  “which is a result of works.”  This latter iustificatio is in reference to various places in scripture where God states that he will not justify the wicked (Exod. 23:7). Again Bucer makes the distinction between the prime cause, God’s goodwill, and all other secondary causes, which is similar to Aquinas’s distinction between a prime cause and a middle cause that is enveloped within the prime cause.  God’s goodwill as the prime cause of salvation does not exclude our cooperation in works as secondary middle causes,  only because good works are contingent on God’s goodwill, the prime cause, demonstrating here Bucer’s teaching of Allwirksamkeit. The allowance for cooperation in one’s salvation in this secondary sense does not weaken the prime cause.
That good works act as secondary causes of salvation is clearly demonstrated by Bucer when he interprets a passage that seems to inherently contradict salvation by works, for example, the case of the thief on the cross.  Because in Bucer’s thought works are subordinately necessary for salvation, the thief could enter heaven only if he had done good works inspired by the Spirit, which would then be rewarded. The careless observer may surmise that the thief did not have time to accomplish a good work, for he died there on a cross, unable to do anything but have faith and die, but this is to understand good works in a rather limited way.  Bucer states:
For suppose that somebody lived in the most ungodly manner throughout his entire life. The Lord however converts him at the end of his life just as he did with the thief who was crucified with our Savior. Now as soon as he has embraced the goodness of God by faith, his mind truly burns for the glory of God. If he can do nothing else, he confesses his sins to the glory of God, and urges others to repent. Even if he cannot do it by prayer, he does it by groans and sighs. In this he now has good works according to which he may be justified, that is, for which he may be judged to deserve admission to the inheritance of eternal life. 
Good works are essential for the reward of salvation.
It ought to also be mentioned that good works are not only a secondary cause of salvation but also signs of that very salvation, since good works are usually visible. On this account Bucer states that men can judge believers as just on account of the fact that they recognize the iutitia of the believer.  This is not understood as a justification by men; rather, when believers are inspired by faith toward iustitia , these works of iustitia are seen by those around who can judge those works as righteous.  They give evidence of one’s election.  Justification is a visibly realized doctrine founded on the dictum that “to know the good is to do the good”; to have true faith is to love your neighbor; to contemplate the iustitia Dei is to necessarily pursue iustitia.
Justification is sola fide. The gift of faith, understood as a deeper inbreathing enabling one to contemplate true iustitia, leads a man or woman to pursue this iustitia in works. These works are a product of the gift of faith, which leads Bucer to maintain that believers are saved by works produced automatically from true faith. In this light he defends the Augustinian definition of justification by faith that works through love, stating: “But let no one be offended at his [Augustine’s] describing love of one’s neighbour as the definition of faith, a definition in term of the effect.” 
God chooses to forgive those whom he chooses. As evidence of this election he bestows upon them faith, or the fuller breath of the Holy Spirit. This gift of faith that comes with the bestowal of the Spirit reveals iustitia resulting in a derivative iustitia. Thus it is in this manner that Bucer holds together an imputed and imparted aspect in his doctrine of justification.
285. BRom (1536), 12; (1562), 12; CP 162.
286. BRom (1536), 290; (1562), 322; CP 290; “Now, the death of our sins and the life of God are so perfectly presented that we are said to be buried into death of Christ, incorporated into Christ, clothed with Christ. Therefore, as we can expect no more perfect presentation of Christ, given symbolically for the apprehension of faith, so also it is most inappropriate to repeat our baptism.”
287. BRom (1536), 296; (1562), 330; CP 305.
288. BRom (1536), 292; (1562), 324; CP 294; “Moreover, since the faith whereby we embrace these promises of salvation cannot exist apart from the Spirit of Christ and some degree of communion in the nature of Christ, that is to say, in the life of God and in true virtue, the Lord, the one mediator between God and men who is always in the midst of his people and always at work by his Spirit in the holy ministrations of his word and symbols, has likewise always bestowed through the sacraments together with the imparting of the Father’s favour his Spirit, but according to a set measure which has varied in accordance with the age.”
289. BRom (1536), 120; (1562), 105; “Si quid benefacit, id inde est, quod fi gmentum Dei est, ad bona opera conditus, et ea, quae Deus ipse praeparat, facit, et perfi cit, ut sua iam in nobis dona remuneret.”
290. Augustine, Epistola CXCIV , 19; PL 33, 880; “cum Deus coronat merita nostra, nihil aliud coronat quam munera sua.”
291. BRom (1536), 129–130; (1562), 119; “Nihilominus tamen, cum Deus velit nos sibi bonis operibus ad nostram salutem cooperati, imo etiam eam perfi cere, κ α τ ε ρ γ ά ζ ε σ θ α ι , Philipp. 2. ac ita statuerit nobis secundum nostra facta rependere, fi t etiam suo modo iustifi catio nostri, hoc est, adiudicatur nobis vita aeterna, ex operibus, sed tum, quando iam haec nobis vita Dei, ex gratia Dei, et Christo merito ante conditum mundum adiudicata est per electionem nostri, et propositum Dei ante saecula factum. Ephe. 1. & 3. adiudicata item per fi dem, hoc est postquam credentes Christo, eius iam compotes aliquo modo facti sumus, felici scilicet illo fi dei initio, quod est per spiritum fi liorum Dei, qui arrabo est huius haereditatis. Sunt enim bona opera, huius fi dei et
292. BRom (1536), 130; (1562), 119; “Triplex itaque est nostri iustifi catio, hoc est, trifariam nobis Deus vitam aeternam adiudicat.” For an extended hermeneutical treatment on Bucer’s concept of triplex iustifi catio , see David C. Fink’s article “‘The Doers of the Law Will Be Justified.’”
293. BRom (1536), 130; (1562), 119; “Prima est, qua vitam aeternam nobis destinat, ea constat utique sola ipsius bonitate, et respectu meriti Christi . . . . Altera, qua vitam aeternam iam aliquo modo exhibet, et frui ea donat, donato suo Spiritu, in quo clamamus Abba pater. Haec iustifi catio constat praeterea etiam fi de nostra, sed quam ipsam quoque nobis Deus ex sua gratuita bonitate donat, et suo in nobis spiritu effi cit. Tertia, cum iam re ipsa, et plene vitam aeternam, vel etiam bona, quibus in hac vita fruimur, exhibet, non iam fi de tantum et spe. Ad hanc iustifi cationem concurrunt facta, sed ea ipsa quoque gratuitae bonitatis Dei dona et opera sunt.” In De Vera Reconciliatione [172(v)–173(r)], Bucer talks of a threefold justification as well; however, there, the fi rst justification is understood as the forgiveness of sins, the grace of God, and communion with the Father and Son; the second justification corresponds to the new life of the believer lived with love and virtues; and the third is the justification by which good works are confi rmed and rewarded.
294. On this point Fink believes that the Scotist-Occamist aspect of acceptio divina is infl uencing Bucer’s thought, in that there is such a close association between election or predestination and justification. He states, “Whatever Bucer’s relationship to the Thomistic theology of his former Dominican order may have been, there can be little doubt that for him, ‘the heart of our salvation, that is, our justification,’ derives from an act of will on the part of God . . . .Justification is thus primarily God’s act of acceptance” (“‘The Doers of the Law Will Be Justified,’” 508–509).
295. BRom (1536), 130; (1562), 119; “prima et per se totaque causa est, sola Dei bonitas, et Christi meritum . . . ”
296. BRom (1536), 218; (1562), 232; “prima iustifi catione, hoc est, ab omni impietate absolutione . . . ”
297. Bucer’s use of secundaria hints not to a following ( secunda ) but to an inferior or second-rate justification that highlights the superiority of the first. In the Expositio that precedes this discussion, he correlates this secondary justification to James, stating: “Illud Iacobi dictum est de secundaria iustificatione, quae consequitur opera, non de primaria et substantiali, de qua hic Paulus.” BRom (1536), 213; (1562), 224.
298. BRom (1536), 218; (1562), 232; “secundaria, quae iusta opera fi t.”
299. ST 1a q.19 a.7; “voluntas Dei, cum sit causa prima, non excludit causas medias.”
300. See also BRom (1532), 401; (1562), 461; CP 146; “But a fi rst cause does not preclude the functioning of second causes. God does indeed act in us in everything, and acts upon us too according to his good pleasure, but he does so in such a way that he causes us to act, so that by his action we come to understand an issue, exercise choice, accept or reject, and set our physical powers in motion.”
301. Luke 23:43.
302. Faith is the chief good work. See BRom (1536), 215–216; (1562), 229; “Credere sane opus est, et omnium bonorum operum caput, quare Dei opus, hoc est.”
303. BRom (1536), 119; (1562), 105; “Nam fac esse, vixerit aliquis impientissime per omnem vitam, Dominus vero in extremo eum ad se convertat, ita ut fecit latroni, qui fuit crucifi xus cum servatore, iam simul atque is fi de dei bonitatem amplexus est, mens eius vere ardet in gloriam dei si nihil aliud potest, confi tetur tamen in gloriam Dei sua peccata, hortatur alios ad poenitentiam, etiam si oratione nequeat, facit id gemitibus et suspiriis. Sic iam habet bona opera, secundum quae iustifi cetur, hoc est, pro quibus iudicetur, esse ad haereditatem admittendus vitae aeternae.”
304. See Barnikol, “Bucers Lehre,” 123–126.
305. BRom (1536), 10; (1562), 10; “Non quod nostri iustitifi catio hominum quoque iudicio nitatur, verum quod quos Deus iustos habet, id est, quibus peccata remittit, hos simul suo spiritu ita adfl et, ut soli ex animo iustitiae studeant, et quamvis in multis continuo et ipsi delinquant, soli tamen quicquid in orbe est iustitiae, obtineant, et hominum iudicio adeo se iustos adprobent, ut cum rem habent cum incredulis, iudicium sibi fi eri orent pro sua innocentia et iustitia.”
306. See BRom (1536), 360; (1562), 412; CP 101.
307. BRom (1536), 22; (1562), 22; CP 196.