Here's your answer;
In order to determine, who is; one need only listen to a person's
testimony of what and who they have placed their faith in! Once
one has heard the "Grace Message" (Paul's Gospel) and have
chosen to place ALL of their faith in Christ as their Lord and Savior,
they are then, sealed and indwelt by the Holy Spirit and baptised
(not by water) into the, "Body of Christ!"
One cannot place some of their faith in their own good works/deeds,
church affiliation, water baptism, and supposed obedience to God's
laws, etc, and the rest of their faith in Christ! They must place ALL
of their faith in Christ alone! We are all lost sinners in need of a
Savior, and that Savior is, Jesus Christ, the Son of God, and God
the Son! It was God the Son who came to this world, in the flesh!