Romans 8:16

The Spirit itself beareth witness with our spirit - Αυτο το πνευμα , that same Spirit, the Spirit of adoption; that is, the Spirit who witnesses this adoption; which can be no other than the Holy Ghost himself, and certainly cannot mean any disposition or affection of mind which the adopted person may feel; for such a disposition must arise from a knowledge of this adoption, and the knowledge of this adoption cannot be given by any human or earthly means; it must come from God himself: therefore the αυτο το πνευμα must have reference to that Spirit, by whom alone the knowledge of the adoption is witnessed to the soul of the believer.

With our spirit - In our understanding, the place or recipient of light and information; and the place or faculty to which such information can properly be brought. This is done that we may have the highest possible evidence of the work which God has wrought. As the window is the proper medium to let the light of the sun into our apartments, so the understanding is the proper medium of conveying the Spirit's influence to the soul. We, therefore, have the utmost evidence of the fact of our adoption which we can possibly have; we have the word and Spirit of God; and the word sealed on our spirit by the Spirit of God. And this is not a momentary influx: if we take care to walk with God, and not grieve the Holy Spirit, we shall have an abiding testimony; and while we continue faithful to our adopting Father, the Spirit that witnesses that adoption will continue to witness it; and hereby we shall know that we are of God by the Spirit which he giveth us.