There are certain “Christian” belief systems that assert that the Trinity is a man-made distortion of who God is. They assert that Jesus is either: a creation of God, an exalted Angel, a demiurge, a mode or expression of the one God. They assert much more, and there are many more views of who Jesus is, that are under or beyond whom Jesus really is as disclosed in Scripture. Conversely, it is those who make such assertions about who Jesus is, whom preach a different gospel — since Jesus is the gospel. And if we get who Jesus is, wrong, then we get the Gospel wrong. If Jesus isn’t the second person of the Trinity, then we end up with a gospel that necessarily starts with man. It took God assuming humanity to himself to bridge the gap between sinful man, and a holy God. If Jesus is just a creation, or a mode, then he is unable to bridge this gap … since He “really” cannot represent us before God — this requires a God-Man. Thankfully Jesus is the second person of the Trinity, which makes the Gospel a predicate of the Trinity. The Gospel is necessarily Trinitarian. Vanhoozer says this way more succinctly than I:
In sum, the Gospel is ultimately unintelligible apart from Trinitarian theology. Only the doctrine of the Trinity adequately accounts for how those who are not God come to share in the fellowship of Father and Son through the Spirit. The Trinity is both the Christian specification of God and a summary statement of the Gospel, in that the possibility of life with God depends on the person and work of the Son and Spirit. The doctrine of the Trinity thus serves both as an identification of the dramatis personae and as a precis of the drama itself. “He is risen indeed!” (Kevin Vanhoozer, “The Drama of Doctrine,” 43-44)
This illustrates my point above, that in order for man to truly be brought into the presence of a holy God, requires that God bring us into his very life! Which He did, in Christ. Unfortunately, this means that LDS, Jehovah’s Witnesses, Apostolic Oneness Pentecostals, Unitarians, et al. all preach a different gospel than the one proclaimed by the Apostles in the New Testament. The Gospel is exclusive by definition, to Trinitarians.