Probably the Trinity has been the source of more bloodshed over the last
1500 years than almost any other single idea. It was political power
that crushed the Arian to begin with, and over the centuries maintaining
the dominance of such a bizarre concept required continued bloody
repression. England hanged a man for unitarianism in 1695, the last
church-based execution there.
Milton pretty clearly adopted the Arian position, along with several
other "heretical" notions (e.g., mortalism). Probably that is at least
one factor which makes his verse such a breath of fresh air in
comparison to Donne, Herbert, Crashaw.
P.S. Milton in his youth tried very hard to be an orthodox xtian, but
the success of The Navity Ode in contrast to the mess he made with The
Passion perhaps indicates that his heart wasn't in it even then.