Deuteronomy 32:5

What is the secret mark which infallibly indicates the child of God? It would be vain presumption to decide this based on our own judgment; but God’s word reveals it to us, and we can proceed confidently where we have revelation to be our guide. Now, we are told concerning our Lord, “to as many as received him, to them gave he power to become the sons of God, even to as many as believed on his name.” Then, if I have received Christ Jesus into my heart, I am a child of God.

That reception is described in the same verse as believing on the name of Jesus Christ. If, then, I believe on Jesus Christ’s name—that is, simply from my heart trust myself with the crucified, but now exalted, Redeemer, I am a member of the family of the Most High. Whatever else I may not have, if I have this, I have the privilege to become a child of God.

Our Lord Jesus puts it in another form. “My sheep hear my voice, and I know them, and they follow me.” Here is the matter in a nutshell. Christ appears as a shepherd to his own sheep, not to others. As soon as he appears, his own sheep perceive him—they trust him, they are prepared to follow him; he knows them, and they know him—there is a mutual knowledge—there is a constant connection between them.

Thus the one mark, the sure mark, the infallible mark of regeneration and adoption is a wholehearted faith in the appointed Redeemer. Reader, are you in doubt, are you uncertain whether you possess the secret mark of God’s children? Then do not let an hour pass until you have said, “Search me, O God, and know my heart.” Do not trifle here, I implore you! If you must trifle anywhere, let it be about some secondary matter: your health, if you will, or the title deeds of your estate; but about your soul, your never-dying soul and its eternal destinies, I beseech you to be in earnest. Make your eternal state sure.