The statement that best 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 choice directly references the specific vision described by Constantine, where he saw a cross, which is a recognized symbol of the Christian faith, implying that he interpreted the vision as a divine endorsement from the Christian God.