Whosoever believing Jesus is the Christ is or has been born of God !
1 Jn 5:1
Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him.
Again, believing on Jesus Christ, His Person and Offices and Saviour Vocation, is evidence that one has been born of God. See one doesnt believe in Christ to become born of God, but quite the opposite, but he believes because he has been born of God.
Paul basically says the same thing here 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:
Notice the order here, sanctification of the Spirit [that's when one is born of God the Spirit and set apart] and belief of the Truth or Faith, that's when one believes ! Its plain that belief of the Truth is resultive of sanctification of the Spirit, hence, he that believes has been born of God.
The sanctifying work of the Spirit in bringing a sinner to belief of the Truth, is a Sovereign work of Grace, so its of faith so to be of Grace.