We walk by faith, and thus it is by faith that we know we are in the body of Christ. We believe in the evangel set forth by Paul: the death for sin, entombment, and resurrection of Jesus Christ. (Thus, we reject the Trinity and Human Free Will.)
But did your know that our gracious God has given us a way to PROVE that we are in the faith? Yep. It's right there in the first chapter of Philippians.
Let's check it out.