Would think the obvious, Jesus. But Jesus didn't write anything. Or at least, nothing we know of (having gone to school as a child presumedly he would have written things.) Plus, because Jesus was Jewish and a rabbi (teacher,) and Christianity as defined by the existence of a Bible didn't exist until decades and centuries after his death, who deserves the credit for creating Christianity?
Christianity was/is a grass roots phenomenon. It also appears that was Christ's own vision of how it should take place. Jesus spent a lot of time traveling and speaking to those who were illiterate, stating that the coming Kingdom could be compared to a a woman kneading yeast into dough. The bit of yeast would grow until it permeated all it touched. The same theme is seen in the parable of the sower and the seed--where the fruit of fertile seed increases more than a thousand fold.
What happens when some (the greedy) see power unfolding? They want to harness it (or at least some of it) for themselves and their own purposes. I am not sure that Constantine was necessarily one of the entirely greedy ones--more that he saw that it might take less energy to work with a growing tide rather than fight against it.
Christ deserves (and receives) the credit for creating Christianity. Who can be accused of using it for their own selfish purposes? Who can be credited for keeping it in align with Christ's teachings about repentance and living in accord with the will of the Father?
Christ was the sower; others watered (with both spoken and written words); others carried the seed to new places; others reaped. The harvest naturally attracted thieves.