If you haven't gotten a load of the very enriching discussion between Dr. Lane Tipton and Dr. Michael Horton on "union with Christ" over at the Reformed Forum, you can get it here.
Dr. Horton's position, as ably analyzed and recognized by Dr. Tipton, owes much, if not primarily, to a sort of "speech act theory" applied to justification wherein God's illocutionary act of declaring the sinner as justified is the "ontological ground" of the subjective perlocutionary effect in the believer. Contra this position, Dr. Tipton argues that the sole ground of the believer's justification is not a floating fiat but union with Christ. In other words, the application of redemption in a believer's present, time-and-space existence (ordo salutis) is founded upon (or united to) the accomplishment of said redemption by Christ in His life, death, resurrection, and ascension (historia salutis).
Justification was the point of discussion, and so it must be said that as Christ Himself was justified, so the sinner, united to Christ by faith through the Spirit, is also justified through the imputation of the former's righteousness.
We can see, clearly evinced, in the Puritan Thomas Goodwin the same kind of aversion to a notion of "speech act theory" and an exaltation of the person and work of Christ in the following:
We must conceive, that the promises of forgiveness are not as the pardons of a prince, which merely contain an expression of his royal word for pardoning, so as we in seeking of it do rest upon, and have to do only with his word and seal, which we have to show for it; but God’s promises of pardon are made in his Son, and are as if a prince should offer to pardon a traitor upon marriage with his child, whom in and with that pardon he offers in such a relation; so as all that would have pardon, must seek out for his child; and thus it is in the matter of believing. The reason of which is, because Christ is the grand promise, in whom, ‘all the promises are yea and amen’ (2 Cor. 1:20), and therefore he is called the Covenant (Isa. 49:8). So that, as it were folly for any man to think that he has an interest in an heiress’s lands, because he has got the writings of her estate into his hands, whereas the interest in the lands goes with her person, and with the relation of marriage to her, otherwise, without a title to herself, all the writings will be fetched out of his hands again; so is it with all the promises: they hang all upon Christ, and without him there is no interest to be had in them. ‘He that has the Son has life’ (1 John 5:12), because life is by God’s appointment only in him (v. 11). All the promises are as copyhold land, which when you would interest your selves in, you inquire upon what lord it holds, and you take it up of him, as well as get the evidences and deeds for it into your hands; the lord of it will be acknowledged for such in passing his right into your hands. Now this is the tenure of all the promises; they all hold on Christ, in whom they are yea and amen; and you must take them up of him.
Thus the apostles preached forgiveness to men, ‘Be it known that through this man is preached to you the forgiveness of sins’ (Acts 13:38). And as they preached, so we are to believe, as the apostle speaks (1 Cor. 15:11). And without this, to rest on the bare promise, or to look to the benefit promised, without eying Christ, is not an evangelical, but a Jewish faith, even such as the formalists among the Jews had, who without the Messiah closed with promises, and rested in types to cleanse them, without looking unto Christ the end of them, and as propounded to their faith in them. This is to go to God without a mediator, and to make the promises of the gospel to be as the promises of the law, Nehushtan (as Hezekiah said of the brazen serpent), a piece of brass, vain and ineffectual; like the waters of Bethesda, they heal not, they cleanse not, till this angel of the covenant come down to your faith in them. Therefore at a sacrament, or when you meet with any promise, get Christ first down by faith, and then let your faith propound what it would have, and you may have what you will of him.
There are three sorts of promises, and in the applying of all these, it is Christ that your faith is to meet with.
1. There are absolute promises, made to no conditions; as when Christ is said to ‘come to save sinners.’ Now in these it is plain, that Christ is the naked object of them; so that if you apply not him, you apply nothing, for the only thing held forth in them is Christ.
2. There are inviting promises; as that before mentioned, ‘Come to me, you that are weary.’ The promise is not to weariness, but to coming to Christ; they are bidden ‘Come to him,’ if they will have rest.
3. There are assuring promises; as those made to such and such qualifications of sanctification, etc. But still what is it that is promised in them, which the heart should only eye? It is Christ, in whom the soul rests and has comfort in, and not in its grace; so that the sight of a man’s grace is but a back door to let faith in at, to converse with Christ, whom the soul loves. Even as at the sacrament, the elements of bread and wine are but outward signs to bring Christ and the heart together, and then faith lets the outward elements go, and closes, and treats immediately with Christ, unto whom these let the soul in; so grace is a sign inward, and whilst men make use of it only as of a bare sign to let them in unto Christ, and their rejoicing is not in it, but in Christ, their confidence being pitched upon him, and not upon their grace; whilst men take this course, there is and will be no danger at all in making such use of signs. And I see not, but that God might as well appoint his own work of the new creation within, to be as a sign and help to communion with Christ by faith, as he did those outward elements, the works of his first creation; especially, seeing in nature the effect is a sign of the cause. Neither is it more derogatory to free grace, or to Christ’s honour, for God to make such effects signs of our union with him, than it was to make outward signs of his presence. (Christ Set Forth)