Christians and historians argue for the conversion of Emperor Constantine to Christianity in the year A.D. 312. However, despite the glaring ignorance of both these groups of people (which sometimes overlap), and the plethora of webpages chock full of error and misinformation, Constantine never converted to Christianity!
Constantine never became a Christian! This fact is supported from his having murdered three of his own family members—his eldest son, his nephew, and his brother-in-law—after having supposedly "converted" to Christianity. Do I have to remind anyone what the Bible has to say on this subject?
Constantine was a sun worshiper long before his supposed "conversion," and he remained one long after it. In fact, Constantine was highly superstitious. In order to cover all his bases, he included the God of Christianity among the plethora of gods he already worshiped.
Emperor Constantine modeled his "church" buildings after the pagan temples and Roman state buildings. Constantine introduced temples into Christianity, where none had existed prior, in order to give a legitimacy to it. Every other religion had their temples, including Judaism, and now so would Christianity. He even named each of his temples after dead saints the way the pagans named theirs after their gods. Christianity was effectively married to the Roman culture under Constantine, and it has stuck ever since.
Constantine also introduced the priestly caste, modeled after the Roman state (for clothing) and pagan religions (for rituals). Though the Church began seeing signs of it during the second century, this was the first time that the Church was divided into two classes—the clergy and the laity. Clergy were viewed as more spiritual Christians, while the laity were viewed as lesser Christians. Many different groups have created their own versions of this two-class system, including Dispensationalism, which teaches the classes of spiritual and carnal Christians. Others teach a "second blessing" distinction. All different ways of men trying to make themselves out to be better than their peers.
Constantine gathered pagan relics from all over the empire and decorated his "Christian" temples with them. Under Constantine, Christians received their own sacred temples, their own sacred priestly caste, and their own sacred relics. Priests even began to receive pay for delivering their message. This "Christianity" was modeled after the world. All its worship practices find their origins in the Roman state and the pagan religions. These pagan practices still exist within most denominations to this day! The Church has never recovered from this.
To the flesh, it might have appeared to be a good thing for the Church to suddenly be on top. No more persecution. Money from the empire provided for her. She had become protected. But this is not the way in which we learned the Lord Jesus. This is not what the Scriptures teach us. If you read carefully through each book of the New Testament, you will find that every single book promises believers suffering via trials, tribulations, and persecution. That is what we have to look forward to, which is why Jesus told us to count the cost of what it means to follow Him. Are we prepared to give up everything for Him?
The same is happening once again. While President Donald Trump has accomplished many great things (not only in his first term, but within his first year), nevertheless the protections he is providing for the Church are not necessarily good things. Yes, they appeal to our flesh, but they are contrary to the promises of Scripture. They might look like blessings for believers, but as with Constantine they are more than likely going to turn out to be curses for the Church. Christians who look for this kind of security and glory in it are not very good Christians, if Christians at all. Following Jesus in this life is not about peace, comfort, and security. If that is why you came to Christ, you came under false pretenses through a false gospel, which means your root is most likely extremely shallow and at the first sign of trouble you will eject from the faith.
Because Christians gathered on the first day of the week after sunset—Sunday, and since most of the empire celebrated the Unconquerable Sun on Sunday, Constantine made Sunday a legal holiday to be a day of rest. St. Peter's in Rome reveals a mosaic of Christ as the Unconquerable Sun (Sol Invictus), demonstrating Constantine's affinity for sun worship. His title was Pontifex Maximus, which means "Chief of the Pagan Priests." Constantine functioned as the high priest of paganism to his dying day. He even used pagan magic formulas to protect crops and heal diseases. If he had ever converted to Christianity, this would not be a title he would appropriate.
With all that you learn about Emperor Constantine, not only was he not the first Emperor to convert to Christianity, but he never converted to Christianity. He was a pagan until his dying day. Everything he did to the Church opened the doors for pagans to call themselves "Christians" all the while bringing their pagan practices into the Church and corrupting it from the inside out. The same practices exist within Catholicism today. But other denominations are not exempt because their worship practice is modeled after the Roman state and the pagan religions. The Church today is a far cry from the Church of the early Christians.