History tells us how The Good Book came about.
It also tells us how it survived.
Without Constantine the religion may not have survived at all.
Constantine's story about the vision, similar to St Paul's, and what happened afterwards, is somewhat reminiscent of the stories in Daniel in the Bible. And we read about it in history, not in the Bible.
It looks very much like Jesus chose Constantine similar to St. Paul. Both of them have had a huge impact on Christianity, the same as Jesus himself.
I interpret History different from you concerning Constantine. From what I have studied - Constantine took on the "Christian faith" purely for political reasons. With this "faith", he did not merely stengthen an already existing religeon but he took what he liked and started an entirely new religion which became known as Catholicsim (Roman Catholics). This was a blend of Christianity and Politics. This new system was not tolerant of Christians that opposed Rome and worked tirelessly to eliminate them.
Also to state that Christianity would not have survived without one mans decisions minimizes God.