New Defender's Study Bible Notes
8:9 beginning at the eldest. It was the tradition that, in an execution by stoning, the oldest should cast the first stone, and so on. Thus, in this case, the eldest accuser was the first to be convicted—by the divine accusing finger—that he also had broken God’s law and thus deserved to die. Only Jesus had the right, therefore, to stone her, but He also had the right to forgive her (Luke 5:23-24), and did so—evidently because He knew she was repentant (John 2:25).
8:9 convicted by their own conscience. This is the first of thirty-two occurrences of the word “conscience” in the New Testament. God has given us a conscience to guide our behavior in matters not specifically covered in the Scriptures. But conscience can only be a reliable guide if it is a “good conscience” (Acts 23:1) and a “pure conscience” (I Timothy 3:9). If not followed as guided by the Scriptures (Acts 24:14,16), the conscience can become “weak” (I Corinthians 8:7,10,12), “defiled” (Titus 1:15), “seared” (I Timothy 4:2), or even “evil” (Hebrews 10:22). The conscience is a reliable guide only if constrained by Biblical principles. Note Acts 24:14-16 in reference to how Paul expressed his conscience.