Romans 8:16

The Spirit itself beareth witness
The thing which the Spirit of God witnesses to is,

that we are the sons of God;
which supposes the case in some sense doubtful and uncertain, at least that it is called in question; not by others, though it sometimes is, as by Satan, which need not seem strange, since he called in question the sonship of Christ himself, and by the world who know them not, and by good men, till better informed: but the testimony of the Spirit is not the satisfaction of others, but the saints themselves; who are ready to doubt of it at times, because of the greatness of the favour, and their own sinfulness and unworthiness; especially after backslidings; through the temptations of the devil, and because of their many trials and afflictions. Now this witness of the Spirit is to establish and confirm it; not to make the thing itself surer, for that stands on the sure foundation of predestination, on the unalterable covenant of grace, on union to Christ; redemption by him, the gift of Christ, and continuance of the Spirit; but to assure them of it, and of their interest in it; for the testimony is given "to our spirits"; so the words are read by the Syriac, Arabic, and Ethiopic versions, and by the Vulgate Latin; which reading seems better than

with our spirits;
for our own spirits are no witnesses to ourselves: the Father and Son are co-witnesses of the Spirit, but not our own spirits; the spirits of the saints are they which receive the witness of the Spirit of God, to which it is made; not to their ears, for it is not an audible testimony; but to their hearts, it is internal; to their renewed souls, where faith is wrought to receive it; to their understandings, that they may know and be assured of it; to their spirits, which are apt to faint and doubt about it. Now it is "the Spirit itself" that bears this witness, and not others, or by others, but he himself in person; who is a divine witness, whose testimony therefore must be greater than others, and a faithful one, who will never deceive; for he witnesses what he knows, and what is sure and certain: his very being and habitation in the saints are witnesses and proofs of their adoption; his powerful operations and divine landings persuade to a belief of the truth of it; and by shedding abroad the Father's love in the heart, and by the application of Gospel promises, he causes and encourages them to "cry Abba", Father; which is a wonderful instance of his condescension and grace.