1 Corinthians 8:1-13 NASB (1) Now concerning things sacrificed to idols, we know that we all have knowledge. Knowledge makes arrogant, but love edifies. (2) If anyone supposes that he knows anything, he has not yet known as he ought to know; (3) but if anyone loves God, he is known by Him. (4) Therefore concerning the eating of things sacrificed to idols, we know that there is no such thing as an idol in the world, and that there is no God but one. (5) For even if there are so-called gods whether in heaven or on earth, as indeed there are many gods and many lords, (6) yet for us there is but one God, the Father, from whom are all things and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him. (7) However not all men have this knowledge; but some, being accustomed to the idol until now, eat food as if it were sacrificed to an idol; and their conscience being weak is defiled. (8) But food will not commend us to God; we are neither the worse if we do not eat, nor the better if we do eat. (9) But take care that this liberty of yours does not somehow become a stumbling block to the weak. (10) For if someone sees you, who have knowledge, dining in an idol’s temple, will not his conscience, if he is weak, be strengthened to eat things sacrificed to idols? (11) For through your knowledge he who is weak is ruined, the brother for whose sake Christ died. (12) And so, by sinning against the brethren and wounding their conscience when it is weak, you sin against Christ. (13) Therefore, if food causes my brother to stumble, I will never eat meat again, so that I will not cause my brother to stumble.
We as Christians have the liberty to go to a group of lost people, like say the Roman Catholics, Jehovah’s Witnesses, LDS/Mormons, SDA Seventh Day Adventists, or Muslims, to sit and share the gospel with them. We just have to be careful to differentiate between their faiths and ours. We have to do it without offending them more than is necessary and it has to be clear to everyone else who is watching including believers and unbelievers. If we don’t make certain to point out our differences it looks to the spectators as if we are acquiescing or giving assent to the aberrant beliefs of the lost. We don’t want people coming away thinking that there is no difference. We don’t want other believers to come away thinking that we agree with the false teachings of these other groups. We also don’t want weaker brothers to start adopting false doctrines because of that misconception. I get it, this does not seem to be speaking to eccumenism, but I think the underlying principle of abusing liberty at the detriment to the weak can apply to eccumenism as well.