“It must be admitted by everyone who has the rudiments of an historical sense that the doctrine of the Trinity, as a doctrine, formed no part of the original message. St. Paul did not know it, and would have been unable to understand the meaning of the terms used in the theological formula on which the church ultimately agreed.”
(W.R. Matthews, God in Christian Thought and Experience, p. 180)
It is, however, necessary to hear and believe the original message -> the gospel of the kingdom.