Covenant

"And I will establish My covenant between Me and you and your descendants after you in their generations, for an everlasting covenant, to be God to you and your descendants after you." Genesis 17:7 

This everlasting covenant made for Abraham can only have an affect in the life of those who believe in the Lord Jesus and truly act on their belief. That is, they obey the Word of God.

Being One

"That they all may be one, as You, Father, are in Me, and I in You; that they also may be one in Us, that the world may believe that You sent Me.." John 17:21

From the first day of a person's new birth, the child of God becomes one with God. He acquires His character, thinks like Him, and desires what He desires. Thus the reason his sole purpose is to save souls.