“He came to that which was his own, but his own did not receive him. 12 Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God — 13 children born not of natural descent, nor of human decision or a husband’s will, but born of God.” (John 1:11-13)
God’s true children are those who “receive” Christ. This means that they have believed in the sufficiency of His sacrifice for their sins, and repented from their wickedness.
These do not become true children of God as the result of any person’s effort, but rather, they are born by the will of God.