They're right.
And neither is the word Bible anywhere in the Bible.
Or omniscience.
Or the Incarnation (meaning God in the flesh).
And yet we teach all of these things, because the word form of these concepts do not need to be present in order for the truth of the concept to be clearly implied or spoken of in Scripture.
And it follows logically from the substantiated claims Jesus made about Himself, and the claims God makes about Himself, that the concept of Trinity is taught in the Bible.
