If we as WELS Christians belong to the Universal Christian Church as we confess in the Apostles' Creed, why is it a sin for us to pray with our fellow brothers and sisters who might be of a different denomination or Lutheran synod? Aren't we all Christians?
Your question illustrates the difference between the Holy Christian Church (the invisible Church) and visible churches.
You are correct in noting that we believe that there is a “Holy Christian Church, the communion of saints,” as we confess in the words of the Apostles’ Creed. That Church consists of people throughout the world who have a common faith in Jesus Christ as their Savior. Because God alone knows what is in a person’s heart (1 Samuel 16:7; 2 Timothy 2:19), we call that Church “the invisible Church.”
What is visible to you and me is where people have aligned themselves with their church membership. By joining a church, people have committed themselves to the doctrine and practice of that church. We cannot look into people’s hearts and see what they believe, but people can tell us what is in their hearts by their words—and their actions, by the doctrine they embrace via their church membership.
“Aren’t we all Christians?” you wonder. Individuals and groupings of people might call themselves Christians and yet: deny the inspiration of the Bible, deny infant baptism, reject the real presence of the Lord’s body and blood in the Lord’s Supper, deny original sin, believe in a literal 1,000-year reign of Christ on earth, etc. False teachings like these are serious and, finally, can be threatening to saving faith. This is why Scripture teaches us not to practice church fellowship with those who believe or teach contrary to Scripture (Romans 16:17).
When there is unity of faith with other Christians, then we can express that unity by praying together, receiving the Lord’s Supper together and combining our resources for mission work and educational efforts. When unity of faith does not exist, we refrain from those joint activities.
Refraining from those joint activities does not mean that we consider the people involved to be outside the Christian faith. Again, faith is a matter of the heart. God alone knows who belongs to his Church. Refraining from praying with people, for example, who call themselves Christians, results when we compare their church’s teachings to the Bible’s teachings and find that the teachings do not agree. Refraining might be challenging, but it is the right thing to do. It is a testimony to the truth of Scripture.