1) Did Constantine the Great toll the bell for the end of true followers of the Christ?
And bring in the rein of false Christians ?
Not as far as I know. If he did, I have seen no evidence suggesting that such was the case.
2) Is there a true form of Christianity that still exists ?
Seriously: probably not. I can't think of a single Christian sect (and that includes Jehovah's Witnesses) that has not been heavily influenced by non-Christian cultural elements and traditional practices and traditional teachings. Simple example: church buildings (or kingdom halls, if you prefer). Such were unknown to early Christians and are unattested in the Bible. Simple example: the position of hands during prayer (folded, or palms together, or fingers interlaced, or arms held out slightly from one's side) - again, all "positions" unattested in the Bible. Even our very language - used in religious worship - is chock-full of (etymologically) pagan religious elements.
However, there *are* many Christians who devoutly and sincerely strive to worship God in the manner taught by Jesus. These are true Christians - regardless of the "forms" of Christianity that exist.
Jim, http://www.bible-reviews.com/