I have been invited to attend the wedding of atheists who are making a civil union before a judge. Previously, they have been living together in sin. I do not approve of their lifestyle, and though they are my friends, I am concerned about doing the wrong thing by attending the reception. What should I do?
First, there is nothing immoral or wrong with atheists marrying. Remember, marriage is an institution God created for all humanity. It is not a "Christian" institution, since marriage originated in the Garden and predates both the fall of Man and the appearance of Christ. So long as a couple meets the biblical requirements for marriage (i.e., one man and one woman who are not already married), then the marriage is appropriate, and therefore a Christian may attend the event without fear of sinning.
Secondly, this couple's choice to marry is an improvement over their current circumstances. Currently, they are living in an immoral relationship (i.e., fornication), which a Christian can't condone, but once the couple marries, they will have corrected this error. At that point, you will have much to celebrate, since the sin you disapprove will have been eliminated! Therefore, attending the wedding isn't an endorsement of their sinful lifestylel; on the contrary, it celebrates the end of a sinful lifestyle.
Thirdly, all Christians are called to be "in the world" but not "of the world:"
Being "of this world" means following the unbelieving world's values, beliefs, and conduct. Christians are not to share in these things. Instead, we are to follow Christ's word and reflect it by our words and actions.
On the other hand, Jesus specifically says the Church was to remain in the world for a time. While we are here, the Christian is an ambassador for Christ, Paul says.
Just as our nation's ambassadors are commissioned to live on foreign soil for a time so they can represent our nation in that place, so are Christians called to live for a time in a world that is not our eternal home so we can represent Christ's kingdom to unbelievers.
Furthermore, Christ expects us to represent Him in a loving way. We are not sent to act as judges and critics (as the Pharisees were) nor are we seeking to shame unbelievers for their sins. Rather, we know it is the kindness of God that bring men to repentance, and so we come to represent the Father's love and kindness in the face of Christ. Like any good ambassador, we want to represent the mercy and grace of our King by reflecting the light of Christ, and in so doing, we hope to attract the unbeliever to Christ.
Jesus lived this example for us in the Gospels. Jesus never sinned, yet He made Himself available to sinners on a regular basis. He ate with sinners, the Gospels tell us. He counseled prostitutes. He stayed in the homes of tax collector and other evil men. He was willing to associated Himself with sinners in this way, because Jesus was sent to heal the sick, and as He said, healthy people don't need the services of a doctor.
Similarly, Christians aren't called to restrict our associations to only sinless people - if this were the case, we could have no friends at all! Instead, we are called to live in and among the unbelieving world as witnesses to Christ. To do this, we must associate ourselves with immoral men. In fact, Paul commands us to do this very thing:
Ironically, Paul says the only immoral people we are NOT supposed to associate with are immoral Christians! On the other hand, we are supposed to live among immoral unbelievers, for there is no other way to exist in this fallen world. In fact, Paul says the only way to keep yourself entirely free from from immoral people would be to leave the world altogether. Clearly, this is not our calling.
So in your situation, you should spend as much time as possible around your unbelieving friends, including celebrating their marriage with them. Attend the reception without expressing judgment or seeking to cause them shame. As you celebrate with them, be a witness to Christ. You are Christ's representative to the wedding, and how much better is it that Christ would be represented in that moment than not at all!
Furthermore, you will not be sinning by your attendance, since there is nothing about your attendance that constitutes sin. And you are not "condoning" their sinful choices simply by spending time with them, any more than we endorse the lifestyle of a sinful person by attending their birthday party or visiting them in the hospital. Again, if we tried to limit our associations to only sinless people, we couldn't even associate with ourselves!
More over, imagine how your friends will react when they see loving, sincere Christians enjoying their celebration with them without passing judgment? Undoubtedly, in the past they have experienced the judgmental attitude of pious Christians, who have withheld fellowship and kindness to make a point. What a breath of fresh air it will be for your friends to experience your fellowship and lack of judgment! Perhaps it will lead them to wonder why you are different, and this will lead to a conversation about Christ.
Remember, we are on the earth to serve Christ as ambassadors so we can introduce people to His kingdom. Let's work in purity and love endeavoring to make as many introductions as we can.