Constantine and Theodosius

After Constantine converted and Theodosius proclaimed Christianity the state religion toward the end of the fourth century, Christianity was carried by the Roman empire. Yet the faith spread fastest after the collapse of that empire, and soon all of Europe was Christian.
Judaism was a legal religion in the Roman empire; Christianity was not, at least not until the conversion of the emperor Constantine. The reason for the prohibition and persecution of Christianity was that Christians claimed one God not only for themselves but for the whole world.
The number of Christians increased from around one hundred at the time of Christ's death to around thirty million by the early fourth century, when the Roman emperor Constantine himself converted to Christianity.
There’s no doubt that Constantine was favorable to Christianity. Still, many people mistakenly believe that he not only favored it but that he made it the state religion. He did not. He signed the Edict of Milan, which made it legal to practice Christianity and ordered that the Christians’ confiscated property be returned to them.

Another mistaken notion is that Constantine exercised complete control over the First Council of Nicaea in 325. The primary reason for the council was due to the growing Arian heresy.
Constantine did not fully understand why Arianism was so controversial, and he even endorsed many of Arius’s ideas.
[W]hen Constantine also endorsed Arius’s ideas, there was an uproar that led the emperor in 325 to call the Council of Nicaea (Asia Minor) to settle the issue. After an intense struggle, the Council condemned Arius, declaring the Son to be “consubstantial” with the Father, that is, sharing the same substance.

If Constantine held as much sway over the Council as many claim, then it is a peculiar thing that the Christology he favored was the big loser.

The next anti-Catholic claim is summarized in Mr. Smalley’s quote above: It is the idea that Constantine decided which books belonged in the Bible and that the ones he did not favor were left out.

The Council Fathers discussed many things besides Arianism, including the proper dating of Easter, the validity of baptisms administered by heretics, and more. One issue they did not discuss, however, is which books belonged in the Bible. They drafted a list of canons (ecclesiastical laws) that you can read for yourself here. http://www.newadvent.org/fathers/3801.htm

Finally, there is the claim that Constantine introduced pagan elements into what was “pure” Christianity up to that point. Many Fundamentalists will claim that doctrines like transubstantiation, the communion of saints, or the sacrifice of the Mass were pagan ideas. But all of these teachings and more can be traced back to the time of the Apostles through the writings of the early Christians.