first cousin marriage

Can i marry my first cousin, (he is my mothers sisters son) we r christians, and parents are against our relation. I have seen lot of querys but couldnt conclude.. Can we register our marriage, will it consider as valid? Is there any problem after we register our marriage. Means in future. If parents give any complaint against this what all problems may occur? parents are emotionally blackmailing us. can we get married legally? What are the formalities? Egerly waiting for a reply. Help me.