In what ways did Constantine's recognition of Christianity change the religion?

The truth is that the religion had already changed quite a bit long before Constantine became emperor. Paul and John had fortold a future apostasy from the true faith as had Jesus. The clergy laity had become firmly established by this time. Jesus ha d told his followers that they were not to 'lord' it over one another, but that they were all 'brothers' in fact he specifically warned against taking on titles, such as at Matt. 23:9,10 not calling one another 'father' which the apostasy ignored by using the title for all clergy. At any rate when Constantine made 'cristianity' the state religion, he in effect took on the title 'Ponificus maximus' or pope and became the leader of the church, defining doctrine such as making the final desicion as to whether God is one God or a trinity of gods. He presided at the council of Nicene. The fact that he wasn't babtized didn't seem to bother the bishops. When he supposedly saw the flaming cross in the sky and was told'by this sign you will conquer' he in effect santioned war for christians....something forbidden by Christ and his apostles. The result of all of this has been the most warlike people on the face of the earth...calling themselves 'christian' and the church being heavily involved in politics and secular government rather than the true occupation of true christians, following the command of Jesus in Matt24:14 and Matt.28:19,20 to preach not any human government but God's Kingdom as the only real hope for mankind.

Expanded Answer

I agree with the poster above, however, Constantine DID abolish all Jewish feasts that were being observed by the Church. He changed the calendar to reflect paganism rather than the Jewish roots of Christianity and taught that the Jews were to become separate from the Church of Rome.

Because the Jews were the foundation of the Gentile beliefs of Christ when these changes came in the Roman Catholic Church was able to introduce idol worship, rituals, requiring sins to be confessed to priests rather than in personal prayer. Constantine was the primary tool of the era to transform the foundational beliefs into a "new religion".

It destroyed the truth & forced false doctrine & such on all that it conquered. Tertullian coined the term 'trinitas' which began to infiltrate & corrupt the doctrine of God. Tritheism or trinitarinianism caused Jesus name baptism & monotheism underground. Constantine forced the false pagan trinity on all.

Constantine's recognition of the early Christians in 325 A.D. made Christianity the SOLE religion of the land at the expense of all others, pagan and Jewish, and turned a floundering religious sect into the official institutionalized religion of the Roman Empire, which is probably the reason why it was able to expand so much and is still flourishing 2000 years later.