I only mentioned ecclesiastical Latin because it was ecclesiastical Latin that was used by the Catholic church (not classical Latin, which was the language of the ancient times). There may have been an expression for 'warrior of Christ' that was commonly used by the Catholic church, which also used Latin (ecclesiastical). I would not know this expression, but it seems that miles christi could be it.
I am sorry, I probably should not have mentioned ecclesiastical Latin at all - it has only brought in confusion so far
Anyway, bellator and miles were both used both in classical and in ecclesiastical Latin. I purposely did not use later words for 'warrior' - choosing more common one bellator. Bellator means warrior, while miles means soldier. Just as in Engllish these words can sometimes be used interchangeably.
In other words, take your pick, they are both fine