Constantine didn't add the doctrine of the trinity, as it had existed before him (though it didn't really take its present form until generations after Constantine's death), and the pagan origin of the trinity is a very tenuous claim. The trinity was invented as a way to explain the concept of the Incarnation (which IS a pagan concept) but there isn't really anything analogous to the trinity in known pagan religions. That was a common claim by nineteenth century freethinkers, but it's been debunked very thoroughly. The trinity is a uniquely Christian form of stupidity. Also, Constantine had exactly zero to do with introducing the veneration of the dead or the use of ikons in worship, as those things also did not become popular until after his death and there is no evidence he promoted or instituted them. Constantine is given far too much credit for Christianity, because the evidence shows that he was mostly a blockhead who had no knowledge of Christian issues and simply allowed priests (who had already corrupted Christianity thoroughly) to run rampant.