Question:
What does association mean to Christians? (For example, you have a friend and she is not saved; she says the only way she will listen to the gospel is if you go with her to her Catholic Church.)

Answer:
This is a good question, but what association means to some Christians it doesn’t mean to others! So it seems to me that a better question to ask would be: “What does association mean to me?” And even better it would be to ask: “What does association mean to God?”

The Bible tells us what God thinks of association: “Be not deceived: evil communications [associations] corrupt good manners” (1 Corinthians 15:33). “What part hath he that believeth with an infidel [unbeliever]? …And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be My people. Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you, and will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty” (2 Corinthians 6:16-18). How important it is to have a close association with the Lord, and we can only have a close association with Him if we are saved, and are not closely associated with “evil” and unbelievers.
The fact that your friend will only listen to the gospel if you attend her Catholic church indicates that not only is she not ready to listen to the gospel of God’s grace to sinners, but that she possibly wants to try to persuade you to receive what her church teaches about the way of salvation.
The verses quoted above show that if you go to her church you would sanction (by your presence) her church’s teaching, and you cannot sanction the teaching of salvation by human efforts if you have trusted Christ alone for your salvation. If you go to her church, and possibly hear something that is not according to Scripture, you will be defiled (“corrupted”-1 Corinthians 15:33) by hearing what is said.If your friend will not listen when you try to tell her the good news that Christ died and rose again to save her soul from hell, then show the gospel to her in your life. Show Christ and His love to her by your actions! And if you seek only to please the Lord, He will make you ready to speak to her when she asks the reason you have peace about things now and in eternity. “Sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear” (1 Peter 3:15).