Romans 8:16

The Spirit itself--It should be "Himself" (see on Rom 8:26). beareth witness with our spirit, that we are the children--"are children" of God--The testimony of our own spirit is borne in that cry of conscious sonship, "Abba, Father"; but we are not therein alone; for the Holy Ghost within us, yea, even in that very cry which it is His to draw forth, sets His own distinct seal to ours; and thus, "in the mouth of two witnesses" the thing is established. The apostle had before called us "sons of God," referring to our adoption; here the word changes to "children," referring to our new birth. The one expresses the dignity to which we are admitted; the other the new life which we receive. The latter is more suitable here; because a son by adoption might not be heir of the property, whereas a son by birth certainly is, and this is what the apostle is now coming to.

— Jamieson-Fausset-Brown Commentary (public domain)