If marriage is only between one man and woman and it is God alone who pronounces a couple as married, what is the role of those who attend the wedding ceremony? Why are we even there? We do not ask these questions just to be foolish, but to suggest that in everything we do, we seek for understanding--answers from God that have meaning.
As a couple exchanges vows and commits to each other, although the vows are before God alone, performing vows before witnesses makes the time more meaningful and pointed. Jesus said, "Therefore whoever confesses Me before men, him I will also confess before My Father who is in heaven." (Matthew 10:32) To stand openly before God and men seals those vows in our heart. We see an example of this when Boaz took Ruth to be his wife. He made the confession openly before witnesses. Is this not our proper place as brothers and sisters in the Lord? We do not come to marry a couple, but to witness their vows before God and to give them an opportunity to declare what is in their hearts to each other before God and man.
There are many cultural traditions surrounding weddings, but universally, a wedding is a time of rejoicing. When we are full of joy, we want others to enter into our joy with us. Witnesses come to participate in the joy of the bride and groom. It is their day, but we give them an opportunity to share their joy with us. In so doing, their joy is made richer, and God is glorified because He is the one bringing them together. We also have an opportunity to serve the couple as they begin their journey together. This serving can be expressed in practical ways such as gifts, as well as feasting and rejoicing in the goodness of God. Music and testimonies that glorify God also allow us to participate together in the joy of the occasion.
In our discussion, we are thinking primarily of a bride and groom who are our brother and sister in the Lord and with whom we have a real relationship. This is where our participation has the greatest meaning. Certainly this is not true of every wedding we attend. There are many other situations we may find ourselves in, and we must get God's mind about each one. God does not want us to identify with sin. If a couple is entering into sin (i.e., marrying into adultery by marrying a divorced person), we should not give witness to that union. Only the Holy Spirit can lead us in these things and give us the proper discernment as we keep ourselves in the love of God.