Jesus teaches that the external sin of adultery and the internal sin of lust are both violations of God’s Covenant.
The parallel account of this teaching is found in Mark 9:47-48.
Having discussed the first Kingdom principle—righteousness is a matter of external obedience to the letter of the law and internal obedience from the heart—with the issue of anger, Jesus now applies it to the issue of lust.
Jesus references the seventh commandment (Exodus 20:14), You have heard that it was said, ‘You shall not commit adultery.’ Adultery is not just a sexual sin. It is a violation of God’s institution of marriage. Marriage is the uniting of soul and body of a man and a woman into a single organism. Marriage is the first institution God gave to mankind (Genesis 2:24). Adultery breaks that union. Adultery was defined as a capital offense under Jewish law.
“If there is a man who commits adultery with another man’s wife, one who commits adultery with his friend’s wife, the adulterer and the adulteress shall surely be put to death”
When Jesus was in Jerusalem, the scribes and Pharisees brought a woman who was caught in the act of adultery and tried to entrap Jesus to publicly stone her to death. But He did not take their bait (John 8:1-11). It was clear that this specific law had not been enforced for some time; it was against Roman law for the Jews to apply capital punishment. Further, the law was not being applied properly even in this instance. The law required that both the man and woman be stoned, but the accusers only brought the woman. Jesus gave permission to whichever one of them was without sin to cast the first stone. Thus He upheld the law, while making it clear the more important issue was the heart. Jesus was without sin, so He could have thrown the first stone. Instead He forgave her, but admonished her to, “Go. From now on sin no more” (John 8:11).
As He dealt with the commandment against murder in the previous verses, so He does with the commandment against adultery. Jesus internalizes it to the heart level, teaching His disciples that the sin of adultery is not only committed through a physical act. Referencing His own divine authority (but I say to you), Jesus teaches that everyone who looks at a woman with (sexual) lust for her has already committed adultery with her in his heart.
It is not enough to claim righteousness if you do not actually have illicit sex with someone. If you even look at a woman with lust for her you are already guilty. The physical and external act of adultery can only occur if it first takes place in the heart.
Once again, Jesus is not only teaching that the external sin of adultery makes a person unrighteous and out of harmony with His kingdom. He takes it a step further and says that the internal sin of lust makes a person unrighteous and out of harmony with the principles of His kingdom. In the judgement to come, the book of Hebrews states that the “thoughts and intentions of the heart” will be judged (Hebrews 4:12).
You have heard that it was said, ‘You shall not commit adultery’; but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart.
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