The most fundamental and essential teaching in the Catholic faith

Catechism

The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the "hierarchy of the truths of faith".   

"Now this is the Catholic faith: We worship one God in the Trinity and the Trinity in unity, without either confusing the persons or dividing the substance; for the person of the Father is one, the Son's is another, the Holy Spirit's another; but the Godhead of the Father, Son and Holy Spirit is one, their glory equal, their majesty coeternal" (Athanasian Creed: DS 75; ND 16).        

No such precise authority in the Gospels

"Our opponents sometimes claim that no belief should be held dogmatically which is not explicitly stated in scripture … But the Protestant Churches have themselves accepted such dogmas, as the trinity, for which there is no such precise authority in the Gospels."   Assumption of Mary, Lifemagazine, Oct 30, 1950, p. 51