The sheep follow him, for they know his voice.John 10:4
What is it that provides the infallible evidence that you are a child of God? It is a foolish presumption to answer this by our own judgment; but God's Word reveals it to us, and we may walk confidently when we have revelation as our guide. We are told concerning our Lord, "to all who did receive him, who believed in his name, he gave the right to become children of God."1 So 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 in the name of Jesus Christ.
If, then, I believe on Jesus Christ's name—that is, simply from my heart entrust myself to 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 of becoming a child of God. The Lord Jesus puts it in another way: "The sheep follow him, for they know his voice."
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. And so the evidence, the infallible mark of regeneration and adoption, is a hearty faith in the appointed Redeemer. Reader, are you in doubt, are you uncertain about whether you are one of God's children? Then do not let an hour pass until you have said, "Search me, O God, and know my heart."2 Do not linger here, I warn you! If you must linger anywhere, let it be about some secondary matter—your health, if you wish, or the title deeds of your home. But about your soul, your never-dying soul and its eternal destiny, I urge you to be in earnest. Make certain of this eternal issue.
1) John 1:12
2) Psalm 139:23