The correct response that 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 significance of the vision he reportedly experienced and its association with Christian symbolism, which played a crucial role in his decisions and the establishment of Christianity in the Roman Empire.