Q: You said something happens when people meet together in church. What is that specifically?
A: One true statement is that God is a God of communion. A God of Love. He is a God of relationships. For each of the 3 persons of the trinity there is, revealed in scripture, a definite sense of relationship between them that only happens when they are together. God is creating a race of Children like his only begotten son, and God had a very close relationship with Him.
In Revelation the church comes together around the throne of God and they are said to be myriad and myriad. Paul uses the example of the different living parts of the human anatomy to relate that we are the body that is the church. We are told that we are individual stones being perfectly put together as a place for God to dwell in.
In the Old Testament, the Glory of God enters the temple when the people of God gather together there.
Q: Is the church anyone who experiences, responds, and is sustained by the work of the trinity? I ask because I have catholic friends who have said that I am not a part of the one “true” church. Am I correct in responding to them like that?
A: The Church is the Gathering of God’s people. Without going into too much catholic doctrine, they teach that the One True Church has a historical record back to Peter, who they believe was the first Pope.
The One True Church is the one mentioned in the Bible. People who come to the cross, realize that they need Jesus, and repent. Whatever flavor of Christianity you have is irrelevant. It is being born again of the Spirit of God who moves mightily. THAT is the only way to come to the One True Church. So yes. You are right.