I guess that by Constantine you mean Constantine the Great.
Constantine and Ambrose were not really contemporaries. Ambrose was
only 7 when Constantine died. He was not involved in decisions
regarding the empire.
Ambrose is better known as St Ambrose. He was the bishop of
Milan and one of the Latin doctors of the Church, or 'fathers' of
the Catholic Church. His theology contributed to laying the
foundation of the Catholic doctrine. In payed an important part in
shaping the doctrine of the immaculate conception. He saw virginity
as superior to marriage and Mary as a model for this. His writings
on ethics were also important. He also opposed the Arianism of
emperor Velentinian II.
