Praying with members of other denominations
During the holidays I come into contact with relatives of other denominations. What is the WELS' position on praying with them? For example, at a Catholic sister's home they want to say grace before the meal. Are we not part of the "whole Christian church on earth"?
There is one Christian church (1 Corinthians 3:16; Ephesians 1:23; 4:12). It consists of people who trust in Jesus Christ as their Savior (Galatians 3:26). Because only God can look into the heart (1 Samuel 16:7) and see saving faith (2 Timothy 2:19), we call this the “invisible church.”
Since God alone can see what is in a person’s heart, you and I understand what people believe by their profession of faith. A person’s membership in a visible church (where you can see who the members are) is a profession of faith. Membership implies that the person’s faith and the church’s teachings are the same. From there, it is a matter of comparing the teachings of a person’s church with the Bible.
When there is unity in faith among people and the churches to which they belong, then we can worship together (Hebrews 10:24-25), pray with one another (Acts 2:42) and work together to proclaim God’s word (Joel 1:3; Mark 16:15).
When there is not unity of faith among people and the churches to which they belong, God instructs us to not practice religious fellowship with them (Romans 16:17-18; 2 John 10-11). That includes not praying with them. While it can be painful to implement those instructions when it comes to family relationships, doing so lovingly can demonstrate that God comes first in life (Matthew 10:37-38).
The problematic scenario you described in your question can be resolved if the host of a gathering holds to the practices of his own family altar and does not put others in awkward or compromising situations by asking for their participation. If the host of a gathering offers prayer on his own, others can respectfully follow the guidance of Scripture and their conscience.
Chapter Seven of This We Believe, a statement of belief of our church body, addresses the subject of church fellowship in more detail. This link will take you to that chapter.