When you roll into the middle east and say believe or die. It’s a pretty easy choice.
Rape your women, enslave your children and kill adult males. All with superior weapons and tech. An easy choice to throw on the cross and say I believe.
Uh… That’s literally not what happened though? Christianity had already spread a lot by the time Constantine declared Rome Christian.
…That’s literally not what happened though…
But you are close. Constantine had been dead for a few decades by the time Theodosius I and his boys put out the Edict of Thessalonica. You may be thinking of the Edict of Milan which made Christianity legal.
For a modern comparison, Edict of Milan in 313 is like a when a state decriminalizes marijuana and Edict of Thessalonica in 380 is like when one makes it legal for recreational use and sale. Don’t think about it too much or it breaks down but one’s a much bigger step.