8:16 Spirit itself. Because of the neuter pronoun here, some have wrongly concluded that the Holy Spirit is merely an influence, rather than a person. However, the Scriptures clearly teach that the Holy Spirit is as much a person as either the Father or the Son. He speaks (e.g., Acts 13:2), teaches (John 14:26), reproves (John 16:8), regenerates (Titus 3:5), baptizes (I Corinthians 12:13), leads (Romans 8:14), produces fruit (Galatians 5:22), empowers (Acts 1:8), and does many other things that require personal action, including “bearing witness,” as noted. A very real evidence that we have indeed been saved is this inward witness in our hearts that we are truly born again into God’s family. However, it is possible for the believer to quench this witness through willful rejection of His leading (see I Thessalonians 5:19), even though His presence still remains.
8:17 joint-heirs. The Son has been given all the nations and the uttermost parts of the earth for “thine inheritance” (Psalm 2:8). He created all things and is “appointed heir of all things” (Hebrews 1:2). This is something of what it means for us to be adopted children and thus to share His inheritance! Note also Ephesians 1:11,14; I Peter 1:4; etc.