How would the growth of Christianity have been affected if Emperor Constantine had not become a Christian?

Answer 1

Provided that the emperors which followed Constantine (all of them but one were Christians) would have not been Christians either, Christianity would not have got imperial endorsement, would not have been helped by favourable policies and laws, and would have been made state law and paganism would not have been persecuted.

Answer 2

Galerius issued the Edict of Toleration, permitting freedom of religion throughout the Roman Empire and thereby ending the Great Persecution of the early years of the fourth century. Thus, even if Constantine had remained a pagan, Christianity would have once again been legal, and would have continued to grow. However, it is likely that Constantine would not have enacted the Edict of Milan, providing Christianity with advantages and privileges.

It must be a matter of opinion as to whether Christianity was destinated to outgrow paganism. Either way, without the financial patronage and imperial support that Constantine provided, and with the temples left in peace to minister to the pagans, Christianity would probably never have totally dominated the Roman Empire in the way that it did.