Ephesians 4:5 One Lord, one faith, one baptism; Jude 3 ...exhort you to earnestly contend for the faith once for all delivered to the saints.
In the New Testament faith denotes both the act of believing and the content of what we believe. Faith as our act of believing is personal and subjective. But faith as the content of what we believe is objective.
The one faith in verse 5 is not our personal act of believing; it is the object of our faith. As Christians we may differ concerning various doctrines [such as tongue speaking, tribulation, rapture, etc.], but we all have the one faith. We all believe in the Person of the Lord Jesus and His redemptive work.
We believe that Christ is the Son of God incarnated to be a man, that He died on the cross for our redemption, that He was resurrected on the third day, and that He has ascended into the heavens. This unique faith is held by all genuine Christians. It is through this faith that we are joined to Christ.
As soon as a person comes to believe in the Person and work of Jesus Christ, the Son of God, he is made one with Christ. Before this he was outside of Christ, but now he is in Christ. This Christ is our Lord, our Head, and we are under His authority.
We are members of His Body, and He is our Head.