The statement that most directly assesses why Constantine thought that the vision in the sky the night before the Battle of Milvian Bridge was a sign from the Christian God is:
"Constantine claimed that he saw a vision of a cross in the heavens, and the cross was a known symbol of Christianity."
This statement accurately reflects the historical account in which Constantine reportedly saw a cross with the words "In this sign, you will conquer," which influenced his belief that the vision was a divine endorsement from the Christian God.