Believing on Christ is a Mystery of Godliness !
1 Tim 3:16
And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
This being believed on in the world refers to Christ. Yes, to believe on Christ is an act of Godliness, Titus 1:1
Paul, a servant of God, and an apostle of Jesus Christ, according to the faith of God's elect, and the acknowledging of the truth which is after godliness;
Now its a mystery, that is Faith in Christ, or believing on Him, because its not something man by nature can do. Thats why Jesus says in Jn 6:44,65
No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.
And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
Man can't believe on Christ until he is born again by the Spirit, in which at that time he is given the fruit/gift of Faith Gal 5:22
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
It, Faith in Christ is resultive of the Holy Spirits work of Sanctification 2 Thess 2:13
13 But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
Thats why Jesus says it, believing on Him, is the Work of God Jn 6:29, God the Holy Spirit. It must be given to believe on Christ Phil 1:29
For unto you it is given in the behalf of Christ, not only to believe on him, but also to suffer for his sake;