As they always use to blackmail poorly catechized Catholics, members of the cult of Felix Manalo, his Iglesia use this argument often that surprise Catholics. Having little time to research and poorly catechized, a Catholic usually ends up an anti-Catholic. Here's a very concise explanation about the myth on Constantine.
THE PAGANIZATION OF THE CATHOLIC CHURCH THROUGH CONSTANTINEThanks to Blackie
The Historic Facts:
Is it true that Constantine was a pagan and used his influence to corrupt the Catholic Church?
Raise Up A Child...
Constantine was raised with Christian beliefs by his mother St. Helena although he delayed his baptism until he was on his deathbed. It seems clear that he had enough faith to seek salvation in Christ, knowing that he was facing eternity and his soul was in the balance
Your Young Men Will See Visions...
After seeing a vision in which he was told that he would conquer under the sign of the Christian cross, he adopted it as his standard and his entire army carried it into battle against the pagan general Maxentius and he won control of the empire in 312 AD.
Freedom At Last...
The following year Constantine signed the Edict of Milan (sometimes known as the Edict of Tolerance) that officially ended the persecution of the Church. It's hard to understand how anyone can believe that Constantine, (who died a Christian despite his delay) would have paganized the early Catholic Church.
Julian Doesn't Keep The Faith.
Forty-eight years later, in 361 AD, the emperor Julian the Apostate launched a persecution of the Church in an attempt to bring back paganism. Why would that have been necessary if Constantine had already corrupted the Catholic Church?
History Tells All
Some careful research and study of the first 300 years of Christianity will reveal that Catholic doctrines such as the Eucharist, Apostolic authority, and the pope as the successor of Peter were believed from the very beginning of Christianity and certainly were not the result of any pagan influence under Constantine.
Christianity has been called Catholic in Acts 9:31 (Greek: "ekklesia katholos") and at least from 107AD when Ignatius, the Bishop of the Church in Antioch, used it in one of his letters to another church which indicates that by that time the original Christian Church was already well known as the Catholic Church in order to distinguish Christ's Church from heretical groups. Keep in mind that that same church in Antioch was where we were all first called Christians as recorded in Acts 11:26. (The Faith of the Early Fathers, William A. Jurgens, Collegeville, Minn.; Liturgical Press, 1970. p.25, #65)
Get The Facts: Make Your Own Decision
Other readings that might interest you:
1. 5 Myths about the 7 Books
2. Biblical Evidence for Catholicism