The Spirit itself bears witness with our spirit, that we are the children of God:
The witness of the Holy Ghost is the work of faith, the witness of our spirits the sense of faith wrought. This is better felt by experience than expressed by words, known altogether and only to them which have it. For me to speak of this to them which have it not, were as if I should speak a strange language. The witness is that "we are the children of God." Not that we shall be, or may be, but are. And what though my very name be not written in the Scripture, thou Thomas, thou John? It is not convenient. What a huge volume should the Bible be if every saint's name were there written! It is not necessary, because all particulars are included in their generals; as he that saith, "All my children are here," means every one in particular, though he name them not; so God, that saith all believers shall be saved, means every one as though they were named. And yet the Scripture doth speak in particular (Romans 10:9). When the law saith, Thou shall not kill, steal, etc., every one is to take it as spoken to himself, as if he were named. Why should not such particulars in the gospel be also so taken? True, say the Papists, if you believe you shall be saved; but where does the Scripture say that you do believe? Ridiculous! The act of faith is not set down in the Scriptures, but the object. The faith which I believe is in the Bible. The faith whereby I believe is in my heart, and is not believed (for that were absurd), but known by feeling. We do not believe that we believe, but we feel it (2 Timothy 1:12). If man should witness, or an angel, there might be doubt; but when there is such a witness as is the Spirit we ought not to doubt. If a man of a weak brain were on the top of some high tower, and should look down, it would make him wonderfully afraid; but when he considers the battlements that keep him from falling his fear abates. So fares it with the regenerate when we look on our sins, and so down and down to hell. Alas! whose heart quails not? But when we consider the brazen wall of the love, truth, and promise of God in Christ, we may be assured without fear. Look upon thy defects, but forget not the truth and power of God. Pretend not the testimony of the Holy Ghost without thine own spirit: nor contrarily, for they go together. Faith, repentance, etc., are the testimony of God's Spirit; if from these thy spirit witnesseth, then it is current. But if thou beest a drunkard, a Sabbath-breaker, unclean, etc., and sayest the Spirit witnesseth thy salvation, it is not God's Spirit, but a lying spirit, for such works are of the devil. God's Spirit indeed witnesseth; but the witness is that they which do such things shall be damned.
(Elnathan Parr, B.D.)
Parallel VersesKJV: The Spirit itself beareth witness with our spirit, that we are the children of God: