Jesus Christ is the head of the church, and for his sake God owns and accepts us as his children. Believing this means much more than people generally imagine.
Those who are not familiar with Holy Scripture may find it strange that we can receive blessing simply by believing. They may think faith is not enough of a virtue to earn us such a reward. However, believing in Jesus Christ is not equivalent to believing a story that we were told or that we have read; it means truly receiving him as presented to us by God the Father. We must embrace the Lord Jesus Christ as the one who has paid for our sin to reconcile us to God. We must entirely trust in him for salvation, assured that he has provided all that we need to gain our eternal inheritance.
If we are certain of these things, it will not surprise us that we become children of God simply by believing. Yet we must also remember that faith has no merit in itself; it is not a question of weighing our faith in the balance to assess its value as a virtue. No, we become children of God through free adoption.
If you are looking for the cause of this, I tell you the true source of salvation is in the mercy of God alone in choosing to take pity upon us. This is achieved by means of faith, as we have said before. When all our pride and vain presumption has been taken away and we recognize that we are lost by nature, then we take refuge in the Lord Jesus Christ.
John Calvin and Joel R. Beeke, 365 Days with Calvin (Leominster; Grand Rapids, MI: Day One Publications; Reformation Heritage Books, 2008), 366.