They have trouble explaining this doctrine. In the end it has to be accepted it by faith. Even Catholics will tell you its a paradox.
I've never understood why some paradoxs are OK, and others are not.
[Origin: 1530ÃÂÃÂ40; < L paradoxum < Gk parÃÂádoxon, n. use of neut. of parÃÂádoxos unbelievable, lit., beyond belief. See para-1, orthodox]
Sorry, but I'm a Catholic, and I'm here to tell you that the Trinity is no paradox. It is, however, a mystery, the technical definition of which is a "revealed truth of the faith which cannot be fully grasped by created human intellect".
Frank Sheed (the late Catholic apologist, not the FReeper) very clearly explains the Trinity in Theology and Sanity.