And this is the gospel: that God is three in one. It is only with this God that humans may have any direct dealings. It is only with a God who can reveal Himself to and in us that we have any hope. We can only come to know a God who is simultaneously the Revealer, the Revelation, and the Assurance of receiving the Revelation. This is true monotheism; all other attempts are polytheistic and idolatrous.
This must be so because any attempt at knowing a Unitarian god must have some mediator. And this mediator must either a) be another divine being or b) a human being who is by nature incapable of revealing God truly and in enough fullness to be of any help. In which case we are left with worshipping a god unknown, as in the latter case, which is blind idolatry. Or, we are faced with another god altogether, in which case we are not really Unitarian but polytheistic, and we are no closer to knowing the original God we sought. Thus God is Trinity: three in one.
And this is the gospel: that we know God, or rather, we are known by God. And this Revelation of God is Jesus.