To be Christian is to be Trinitarian. To be anything otherwise and still claim Christianity is to be in a state of damnable error and deception. In fact, the whole of created reality bears the stamp of the-One-and-the-Many as evinced in the universal-particulars relationship inherent in every created object. God's Trinitarian "seal of approval" is emblazoned on creation as it is on redemption.
Robert Letham, a minister in the Orthodox Presbyterian Church and teacher of Systematic and Historical Theology at Wales Evangelical School of Theology, explains how our whole being must possess an utterly Trinitarian thrust in terms of the expressions of our piety—in prayer, preaching, the worship service, and the Sacraments:
"In a chapter of my book, The Holy Trinity, I describe at some length how the worship of the Western Church has been truncated by the comparative neglect of the doctrine of the Trinity. For most Christians—and I include members of Reformed churches—the Trinity is merely an abstruse mathematical puzzle, remote from experience. Despite our reservations about many aspects of the Eastern Church, Orthodoxy in contrast has maintained a pronounced Trinitarian focus to its worship through its liturgy, which has roots in the fourth century. This is no incidental matter; worship is right at the heart of what it means to be Christian and what the church should be doing. The sole object of worship is God. The God who we worship has revealed himself to be the Father, the Son, and the Holy Spirit, three distinct persons in indivisible union. I have argued elsewhere that this is his New Covenant name (Matt. 28:19–20). It follows that our worship in the Christian church is to be distinctively Trinitarian. Yet if we were to thumb through any hymnbook, we would be hard pressed to find many hymns that contain clearly Trinitarian expressions, while many of our favorites could equally be sung by Unitarians—think of 'Immortal, invisible' or 'My God, how wonderful thou art.' As for the average person in the pew, why not try a random survey next Sunday—ask a random selection of half a dozen people what the Trinity means to them on a daily basis, and see what results you get? Then compare your findings with the words of Gregory of Nazianzus, who wrote of 'my Trinity' and 'when I say God, I mean the Father, the Son, and the Holy Spirit.'
If this problem is as real as is generally recognized but yet as important as I have presented it, how do we go about seeking to redress it? There are no easy, slick solutions. This is not a matter to be resolved by a quick twelve-step program or in an adult Sunday school class. It will take much thought, careful teaching, and a concerted plan to put right what has for so long been askew—since I argue this has been a problem for centuries, with notable exceptions, at least since Aquinas. What is needed is to instill in our congregations a mindset directed, as of second nature, to think of God as triune. From there will come ripple effects on the way we think of the world around us, and of the people with whom we mix. What we need is to develop a thoroughly Christian view of God, the world, the church, ourselves, and others.
The first, and indispensable, steppingstone is ourselves as leaders of the church, and in particular those who are ministers of the Word. It is of the utmost importance that we saturate our minds with reflection and meditation on God, for we stand in the pulpit as no less than his representatives in speaking his Word. It means our consistently contemplating God in Trinitarian terms. John Stott has been accustomed to begin each day with a threefold greeting to the Holy Trinity; how far are your own prayers and thoughts of God shaped in this way? It takes disciplined thought and prayer, consistently day in, day out deliberately to think of God biblically, theologically, and ecclesially as triune. As leaders of the church, you are called by God to do this. You cannot expect the congregation committed to your charge to follow suit unless you are leading the way. It means your being shaped and driven not by some man-made purpose or by the concoctions of management gurus but by the truth of the triune God himself drawing and molding you.
There are definite and particular ways in which your congregation can be taught to develop its grasp of the Trinity. The first such avenue is in your preaching and teaching. How often have you preached on the Trinity? The Church of England, in following the church year, has Trinity Sunday the week after Pentecost; this can provide an opportunity to draw attention to the Trinity at least once a year, as Advent is a reminder of the incarnation, Good Friday of the atonement, Easter Sunday of the resurrection, and Pentecost of the coming of the Holy Spirit. However, this is a bare minimum—just about starvation rations. Perhaps a short series may help, providing it is not something that is forgotten as you move on to other things. Much better is, on top of that, to refer consistently to God not always as 'God' or 'the Lord' but as 'the Father, the Son, and the Holy Spirit,' always bearing in mind that he is three in indivisible union.
The same principles apply to praying as to preaching. You may not be able to preach on the Trinity every week—it would be unbalanced if you did!—but you can pray every week. When you pray, pray 'Our Father in heaven.' What an amazing way to address God! It means that we, through Christ the Son, have been granted by adoption the same relationship to the Father that he has by nature! It immediately throws us into the context of prayer to the Father by the Holy Spirit (see Rom. 8:26–27) through the mediation of Christ the Son. We should bring this to expression regularly in our public prayers. We should show the congregation that this is the way we pray. We should show them that in prayer we are saturated in a Trinitarian atmosphere, given to share in communion with the triune God. We should impress upon our people that in the Holy Spirit, God the Trinity has come to dwell with us, indwelling—better, saturating—us and making his permanent residence with us (John 14:23).
This leads us to the nature of church worship and the structure of the service. In all the works of God he takes the initiative. He created in accordance with his free and sovereign will; no one was there to advise him. In grace, the Son became incarnate 'for us and our salvation'; this too was the result purely of the grace of God, undeserved, unprompted. In our own experience, God himself brought us to new life by his Spirit; our faith and repentance is a response to his prior grace. We love him because he first loved us. Is it any different in worship? Is that primarily something we do? No, first of all God goes before us. He has called his church to himself. He is there to greet us. As we gather, it is to meet with him, but first he has drawn us. Moreover, our acts of worship are accepted because they are offered in union with Christ. He, in our nature, is at the Father’s right hand. From this it follows that the elements of worship are a dialog in which the holy Trinity takes the initiative. Through his ordained servant, the Father through his Son by the Holy Spirit calls us to worship. He speaks to us in his Word read and proclaimed. He receives our praise and prayers. He communes with us in the sacrament. In the benediction, he dismisses us with his blessing—which is far from a pious wish or prayer that such things might be, if it is the will of God. Rather, the benediction is a declaration of a real state of affairs, undergirded by his covenant promises. This is a dynamic view of worship, one that follows squarely in the Reformed tradition and is rooted in biblical teaching. Our congregations need to hear it, they need to understand it, they need to imbibe it and be permeated by it. At my previous church, our regular bulletin expressed this. Periodically we would draw everyone’s attention to it and sometimes produce a written two-page memo explaining it, so as to keep it fresh in mind.
The call to worship is a good place to begin. I often use a congregational response to the call. It is based on Ephesians 2:18, where Paul says 'For through him [Christ] we . . . have access by one Spirit to the Father.' These words impress on the mind the point that our worship can only be Trinitarian. So too does the famous passage in John 4:21–24, where Jesus says that those who worship the Father must worship in spirit and in truth. Every occurrence of pneu=ma (pneuma, spirit) in John, except two, is a reference to the Holy Spirit, while the truth is consistently a reference to Jesus (John 1:9, 14, 17; 14:6). Hence, acceptable worship of the Father is in the Holy Spirit and in Christ, the Son. It is important that this is stamped upon the service right from the start. Christian worship is worship of the holy Trinity, nothing less.
The church where we now attend has, immediately after the call to worship, a short Trinitarian doxology which the congregation sings in response; it is varied from time-to-time so as not to get monotonous. Then the first hymn is very often, if not invariably, Trinitarian, a practice I have come to use myself as often as I can. Calvin thought this was the most appropriate way to begin too, so we are in good company. However, as I remarked, there is a considerable lack of explicitly Trinitarian hymns. Many from the ancient and medieval church have this focus. Our former music director in Delaware, Peter Merio—a graduate of the Sibelius Academy in Helsinki, who also taught there—brilliantly arranged one gem from the fifth century that we dug up from the English Hymnal, edited by Ralph Vaughan Williams in 1933; but there are very few in Reformed circles with his capabilities. Some recent favorites try hard but fall into heresy—an ever-present danger in this area. The hymn 'There Is a Redeemer,' which I have heard sung in the OPC, is generally excellent but has a refrain, 'Thank you, O our Father for giving us your Son, and leaving your Spirit till the work on earth is done.' The Father does not leave the Holy Spirit; the Eastern and Western Churches divided over arguably less.
We have looked at preaching and teaching, prayers, the call to worship and benediction, hymns; there remain the sacraments. Baptism is into the name of the Father, the Son, and the Holy Spirit. Dare anyone say the Trinity is a recondite matter for advanced philosophers when every single member of the Christian church has the name of the Trinity pronounced over him or her? According to Matthew 28:18–20, it is the foundation for Christian discipleship. Similarly, in the Lord’s Supper we receive and feed on Christ really and spiritually; this is by the Holy Spirit who makes the sacraments efficacious. Moreover, since the works of the Trinity are indivisible, in feeding on Christ by the gracious enabling of the Holy Spirit, we are given access to the Father in the unity of the undivided Trinity.
In short, every aspect of Christian worship is an engagement with the Trinity or, rather, a way in which the Trinity engages us. As leaders of Christ’s church, we have the indescribable privilege of leading his people into the realization of something of what this entails. It is a task far beyond our capacities; we are utterly ill-equipped to deal in such transcendent matters. The Bible records that, when given a revelation of the veiled glory of God, human beings are brought to their knees, overcome, broken (e.g., Isa. 6:1–5, Ezek. 1:1–3:15, Acts 9:1–9, Rev. 1:9–18). Yet in his grace, our God has admitted us to fellowship, communion, and union with him as his adopted children, so that we are being transformed from one degree of glory to another by the Spirit (2 Cor. 3:18). The Father and the Son have made their permanent residence with us in the person of the Holy Spirit (John 14:15–23). As ministers of the Word, we have been co-opted as instruments by which the Christ’s flock is changed into his image by the Spirit so that Christ will be the firstborn among many brothers. Doesn’t that thrill you? Doesn’t it make you want to know him better? Doesn’t it impel you to develop a mind shaped by the knowledge of the Father, the Son, and the Holy Spirit and to lead your congregation on to that goal, too?" (Developing a Trinitarian Mind, originally published electronically in Ordained Servant August-september 2008)
Also, Dr. Carl Trueman lectures on the Trinitarian Nature of Devotion here.