You don't have to fight those who don't believe you are Christ's. You have nothing to prove. "For though we walk in the flesh, we do not war after the flesh:For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds; Casting down imaginations, and every high thing that exalt itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ. Do ye look on things after the outward appearance? [No!] If any man trust to himself that he is Christ's, let him of himself think this again, that, as he is Christ's, even so are we [you] Christ's. But he that glory, let him glory in the Lord. For not he that commend himself is approved, but whom the Lord commend.(2 Corinthians 10:3-5, 7, 17, 18). Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ (Philippians 1:6). For it is God who works in you, both to will and to work for his good pleasure (Philippians 2:13).