It's actually a very good thing that this is all you found regarding Constantine's influence because most of the current "information" put out by modern Evangelicals concerning his influence is grossly overstated and is based more on the Da Vinci Code than history. Anywho, you are right in that alot of Catholic doctrine has been developed over time. However it is, as you point out, based in history. I think that in most cases Mary's importance is taken too far but she, along with the other saints, were venerated and honored from the get go. And regarding the Eucharist, I currently do not subscribe to transubstantiation yet on the other hand the Early Church viewed it as much, much more than merely symbolic (i.e. God is, to use your terms, mysteriously present in the meal though a hard and fast doctrine wasn't developed until later.) I guess my point is simply that though I disagree on much of the semantics concerning Catholic doctrine, I do believe their assertion that their lineage goes back to the beginning is legitimate.