Do you feel called to go into the military, or would you be doing it just to get your schooling paid for?
Military life IS a calling. It is not easy. The atmosphere is far from Christian and there are difficult physical and emotional demands.
If you are doing the military because you feel called to it, and to serve your country then when you are 18 go ahead and enlist. If you are only doing it to get college paid for, I think there are more appropriate options.
You can work and go to school. You can also work to improve your grade point average. Realistically, no matter who pays for your schooling you are not going to get a PhD with a 2.7 GPA. If a PhD is your goal, put effort into improving your studying methods, commitment to schooling, get tutoring if need be, etc etc.
My husband worked while going to college (and raising a family) and did get his PhD. it took longer than it might have for a single man (though several of his unmarried friends took just as long) but he achieved his goal.
Is a Christian/Catholic college a necessity? or is it merely a preference. I ask this not because I feel it is not a good idea, but if a Catholic atmosphere is very important to you, you are not going to get that by entering the military.
I suggest making an appt with an admissions counselor at one of the schools you are interested in and discussing tuition options or credit transfers with them. Speak to your own school academic counselor as well.
Don’t go on hunches and assumptions etc, or hearsay. Get real facts so you can make a solid decision.