Did Constantine invent the Catholic Church?

No. Emperor Constantine legalized and supported Christianity in the fourth century, but the Catholic Church had already existed for nearly three hundred years.

What Constantine actually did. Christianity suffered periods of Roman persecution before Constantine. In AD 313, Constantine and Licinius issued the Edict of Milan, which granted religious toleration and restored confiscated Christian property. Constantine later summoned the Council of Nicaea in AD 325 to address the Arian crisis. The bishops, not Constantine, judged the doctrinal question and confessed that the Son is truly divine. Constantine influenced the Church's public circumstances, but he did not invent her faith, hierarchy, or sacraments.

The Church before Constantine. The New Testament already describes a visible Church devoted to apostolic teaching, fellowship, the breaking of bread, and prayer (Acts 2:42). It names bishops and deacons, records councils, and teaches the laying on of hands for ministry (Acts 15:1-29; Phil 1:1; 1 Tim 4:14). St. Clement of Rome wrote about apostolic succession around AD 96. Around AD 107, St. Ignatius of Antioch used the expression "Catholic Church" and taught that Christians should remain united with their bishop.

Catholic worship was already present. Around AD 155, St. Justin Martyr described Sunday worship with readings from Scripture, prayers, an offering of bread and wine, thanksgiving, Communion, and assistance for the poor. He also taught that the Eucharist is the flesh and blood of the incarnate Jesus, not ordinary food. His description closely resembles the structure of the Catholic Mass (CCC 1345). Around AD 180, St. Irenaeus defended apostolic tradition, the succession of bishops, and the special importance of the Church at Rome.

What about Nicaea and the Bible. Constantine did not decide that Jesus was God, and the Council of Nicaea did not create the biblical canon. Christians worshiped Jesus as Lord from the apostolic age (Jn 1:1; Phil 2:6-11), and the Church was discerning the canon long before and after Nicaea. The council defended an inherited belief against a new distortion. When someone claims Constantine invented Catholicism, ask which Catholic belief supposedly began with him, then compare that claim with Scripture and pre-Constantine Christian writings.