Why did Constantine declare Christianity legal?

Answer 1

Constantine's Edict of Milan in 312 proclaimed universal religious freedom in the Roman Empire. In 313, Constantine ordered that anyone who had confiscated property from the Christian Church during the persecutions of the previous decades must return it immediately to the same churches and pay compensation for any damages. But this only applied to the largest and best organised Christian group, who had adopted the term "Catholic" - it did not apply to other Christian groups that followed Gnostic Christianity. Constantine had family connections to the Christian Church but, more importantly, he saw orthodox Christianity as serving his interests as a uniting force in the Empire.

Answer 2

Constantine wanted to get on the side of any gods real or not real, just to be on the safe side. When he realised that the God from Christianity might actually be real, he declared Christianity legal.