Barth argues that the Trinity is not a challenge or a qualification of monotheism, but the only true form of monotheism.  Antitrinitarianism always collapses either into the denial of God’s revelation or of God’s unity.

Denial of revelation because “To the degree that it maintains the unity of God it has to call revelation in question as the act of the real presence of the real God.  The unity of God in which there are no distinct persons makes it impossible for it to take revelation seriously as God’s authentic presence when it is so manifestly different from the invisible God who is Spirit.”

Denial of unity because . . .

“to the degree that it is ready to maintain revelation but without acknowledging the substantial equality of the Son and the Spirit with the Father in heaven, the unity of God is called into question.  In its concept of revelation it will not in fact be able to avoid interposing between God and man a third thing which is not God, a hypostasis which is not divine - it does not want that - but semi-divine; it cannot avoid making this an object of faith,” with the result that revelation, though not being God, is crudely or subtly deified.

In sum, “If revelation is to be taken seriously as God’s presence, if there is to be a valid belief in revelation, then in no sense can Christ and the Spirit be subordinate hypostases.  In the predicate and object of the concept revelation we must again have, and to no less a degree, the subject itself.  Revelation and revealing must be equal to the revealer.  Otherwise there is no room for them beside the revealer if this be the one God.  The unity of God would render revelation and revealing impossible.  Christ and the Spirit would not just be foreign to and totally unlike the Father, as Arius said in dangerous proximity to a denial of all revelation.  They would have no more to do with Him than any other creatures.  Only the substantial equality of Christ and the Spirit with the Father is compatible with monotheism.”