This is a critical subject because the way this scripture is believed will determine whether one has an open mind or not. Any religion can claim that they are the 'one faith' mentioned at Ephesians 4:6. Making such a claim does not mean that a particular religion is the so called true religion.
The question is, what constitutes the true faith? Is it a religion or a way of life? Jesus made an interesting comment at John 4:21-24. He said: "Jesus declared, "Believe me, woman, a time is coming when you will worship the Father neither on this mountain nor in Jerusalem. ( The Samaritans worshipped their God on Mount Gerizim and the Jews worshipped their God on Mount Zion. Jesus's statement was very telling since he said that God would not be worshipped on either mountain. That is significant since the Jews believed that they had the true religion.)"You Samaritans worship what you do not know; we worship what we do know, for salvation is from the Jews (notice Jesus said salvation is from the Jews, he didn't say the Jews were the true religion) "Yet a time is coming and has now come when the true worshippers will worship the Father in spirit and truth, for they are the kind of worshippers the Father seeks. God is spirit, and his worshippers must worship in spirit and in truth."
Again Jesus made it plain that the Father was seeking worshippers to worship Him in spirit and in truth. There is no indication from these words that being of a particular religion would result in finding favor with God. However, an individual could make the effort to worship the Father as the Father would want to be worshipped.
At John 14:6 Jesus said: "Jesus answered, "I am the way and the truth and the life. No one comes to the Father except through me." Where does Jesus mention a particular religion as being the only way for salvation? It is obvious that the true faith is a way of life that a Christian models after the life and example of Jesus Christ. No church, religion, Governing Body, priest or other religious leader can get in the way of that way of life.
So, the one true faith is our faith in Jesus Christ as the one sent by God to pay for our sins. Whenever a religion claims to be the only way to salvation they are standing in the way of and actually replacing Jesus as our only hope for salavation. They are actually putting themselves in the place of the Messiah and are making a claim that only God and his Son can make.