I have the mark of caesar, and legionnaires still attack me on sight. Im not wearing any NCR armor except for the 1st recon beret. Why are they attacking me?

Make sure that Craig Boone isn't in your party. Even half way across the world he will make the legions mad. If he is, then you have to dismiss him and pick him up later (note that even after the legion initiation quest, he will still agro any legionary)

I had the same problem.
Hope it helped!

Yea, guess it was boone. Thanks.

also, it is just a one-time freebie. if you piss them off after you get it they will still attack.