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 statement captures the essence of Constantine's interpretation of the vision and its significance within the context of his affinity for Christianity, particularly in relation to the events surrounding his rise to power.