The statement that most accurately 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 directly references the vision of the cross that he reportedly saw, which became a significant symbol for Christianity and played a crucial role in his conversion and support for the Christian faith.