Christian's what do you think of this Casting Crowns song? Is it slightly sexist?

no... it's only sexist cuz you believe war should be a man-thing only. But actually a lot of women participate in front-line wars too. So imo, the song is just about how we need to be soldiers of Christ, it's not sexist. But I do agree the lyrics can be rephrased slightly better to offend the more sensitive people.