Note: Before reading further, first consider Mt 7:3; 18:15-17, and Eph 4:15 (and other biblical texts concerning proper church discipline).
Every once in a while a fellow believer will say something, something theological, that doesn't sit right with me. When that happens the first thing I do is tell myself to not be casually agreeable...you know, that involuntary head nod that politely tells a person you're tracking with them.
Second, I genuinely listen. Afterall, I can't tell you how many times I've been dead wrong about something. Sometimes it's my own error. Other times, it's the bad theology I learned growing up or at, say, seminary. Whatever the case, I can't learn if I don't listen. In this regard, I want to be like Apollos as I have plenty of Priscilla and Aquila's in my life (see Acts 18:24-26).
Third, I dialogue. And best I can, I try to make counter arguments using only the Scriptures. I do this for two reasons. First, it keeps the conversation from turning into an opinion-fest. Second, if the person becomes offended, their offense is with the Bible, not with me.
By this time the issue is usually resolved. If not, I simply say, "Can you give me some time to think about this? To study the Scriptures regarding this topic?" Then, I go do just that...I study and, if necessary, seek wise counsel. I don't study to show up the other person. I don't study to gather loads of arsenal. Instead, I study to present myself before God as one approved (2 Tim 2:15). Then, in some appropriate way, I present my findings.
If the issue is still unresolved, one of two things happens. If the issue concerns a non-essential theology, I just live in the tension, pray, and continue to study. If the issue is an essential theology, I proceed cautiously, carefully following the Scripture's directives (Mt 18:15-17). Then, I follow the Spirit's leading. Sometimes God directs me to let the whole thing play out. Other times, God directs me to distance myself from the situation. And every once in a while, God directs me to rebuke (rebuking, by the way, doesn't need to be mean-spirited).
But there were also false prophets among the people, just as there will be false teachers among you. They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them - bringing swift destruction on themselves. Many will follow their shameful ways and will bring the way of truth into disrepute. In their greed these teachers will exploit you with stories they have made up. Their condemnation has long been hanging over them, and their destruction has not been sleeping (2 Peter 2:1-3).