My church is in error – what should I do?
If you believe there is hope they might change, or if your conscience demands it of you, then you should carefully prepare your argument and present it privately to the pastor or board of elders (depending on the authority structure of the church). It is better if done in writing, for you have a record of what you said, they can take their time considering it, and it is less confrontational. You should be very respectful in your written approach, wise to pick the issue most likely to succeed, and be very concise in your argument. If the church is very large and has not shown any willingness to reconsider their doctrine or practice, then you should simply leave the church.
Sermon outline about Biblical separation: Biblical Separation.
Sermon outline about sinful compromise: God Hates Compromise.
Sermon outline about being on the Lord’s side: Who Is on the Lord’s Side?