Prayers to the saints and for the dead go back as far as the era of the Catacombs. Of course, the Christians then knew and understood, and Catholics ever since then do know and understand, that saints are to be reverenced and honoured, but that God alone is to be adored. I suppose that you can decide that the church went off track not in Constantine's time, but much earlier, perhaps even in the first generation after the apostles. I have actually heard that argument from one particularly zealous Lutheran. If you want to go that route, however, you must explain why Christ abandoned His church for almost 1500 years and why anything, even the canon of Scripture, that comes down from that time is to be trusted.