If you don't make clear the Trinity in your gospel presentations, here are three consequences...
They won't understand Jesus
Jesus simply is the Christ, the Son of God. That's how all the Gospels identify Him. By definition He is anointed with the Spirit and He is Son of the Father. Jesus is intimately related to the Father and Spirit and cannot be understood without that Trinitarian context.
If God is introduced in single-Person terms, Jesus will appear on the scene - almost by definition - simply as Prophet. Once God has been defined without Jesus, His entrance into the explanation can only ever suggest that He's a lesser being.
If He comes late to the presentation, he is coming to solve a problem that is not really his. Athanasius made much of the Word who made us in the beginning, remaking us in salvation. But modern presentations have a maker on the one hand and a different saviour. This feeds into...
They won't understand the cross
Who is the One on the cross? Is Jesus a third party punished by God? Is God hell-bent on judgement and destruction but this other force with this other will placates Him - almost in spite of Himself? That's precisely how it looks when we begin our presentations unitarianly.
People need to know that God was in Christ reconciling the world to Himself (2 Corinthians 5:19). This is not to deny penal substitution. On the contrary, it's to uphold penal substitution (2 Cor 5:21). As John Stott says in his famous chapter "The Self-Substitution of God", we mustn't make Christ "a third party thrust in between God and us."
At the root of every caricature of the cross there lies a distorted Christology... In particular, it is essential to affirm that the love, the holiness and the will of the Father are identical to the love, the holiness and the will of the Son. God was in Christ reconciling the world to himself. (The Cross of Christ)
The One on the cross is the One who made us. And He is perfectly expressing the love of His Father (John 3:16; Romans 5:8; 1 John 4:10).
So many gospel presentations look like (or even explicitly say that) Christ buys off a reluctant and angry Judge, rather than Christ demonstrating the very love of God in substituting Himself for sinners.
You'll define God as Creator and Judge
What's wrong with that? you might ask. Well God is Creator and Judge, but the creeds speak first of "Father." Before there was anything made, before there was anyone to judge, there was a Father. And He was pouring life and love into His Son by the Spirit.
Foundationally God is life-giving. Yet, functionally unitarian presentations make God out to be, foundationally, Creator and Judge. And His status as Maker is instantly framed in terms of His demands on us. There's a logic that says "God made us, therefore we owe Him." Do you hear what happened there? Creation ought to first make us consider the life-giving, out-going, gracious character of God. But if its spun unitarianly we have a self-focused God who makes in order to get. And what he wants is regularly unpacked in terms of moral effort.
In other words, it begins to sound very much like Islam. God, by definition, lords it over us - that is what it means to be this kind of God. And what does it mean to relate to this kind of God? It can only mean one thing: submission. So the gospel can only be unpacked as "bowing the knee to our Creator and Judge" and salvation is essentially avoiding being crushed by the higher power. In such presentations they might eventually speak of knowing God as Father or of "having a relationship with God", but the whole set-up leaves the listener extremely dubious.
There's bags more I could say, but I'll leave it there. You can add more in the comments if you like. But even if these were the only reasons to do so, they really should move us to present a trinitarian gospel...
Now if only someone would write such a thing...
i f o n l y . . . i f o n l y . . .