Matthew 5:27-30

²⁷ “You have heard that it was said to those of old, ‘You shall not commit adultery.’ 

²⁸ But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart. 

²⁹ If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. 

³⁰ And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.

looks at/ lust for (v.28) = keeps looking at/ keeps lusting for

Without exception, my commentaries that have something to say about verses 29 and 30 say that they aren’t to be taken literally, although Morgan goes so far as to say that anyone who lusts can’t be a Christian.

God forbade adultery in the law. The Pharisees saw adultery as an illegal sexual union. But to refrain from the physical act did not fulfill the spiritual demands of the law, for the law demanded not only abstinence from a physical act but from the lustful desire that would produce the act. The law demanded purity of thought as well as abstinence from an act. Christ warned that the one who lusts is guilty of adultery and has thus violated the law.

Christ warned His hearers to remove the cause of the offense. He was not teaching physical mutilation, for a blind man can lust, and a man with no hands can have unlawful desires. Christ taught that one must deal with the sin of lust because this was the root of the problem of adultery. It is not enough to merely abstain from lust’s outward manifestation, that is, adultery. — Pentecost, page 179

I would agree in principle with Pentecost. Obedience to the law has to be inward and not simply an outward show. Lust is as much a sin as adultery, and this is as true in the age of grace as it was under the law. Paul writes: Therefore put to death your members which are on the earth: fornication, uncleanness, passion, evil desire, and covetousness … (Colossians 3:5).

But I’m not sure the verses aren’t to be taken literally. I think what the Lord was saying was, “If you are relying on the law and  your own observance of it to save you, you’d be better off chopping off the parts of your body that cause you to sin than to end up in hell.”

Of course, His underlying point was that the law couldn’t save because nobody has ever existed (Himself excepted) that could keep the law. Remember back in verse 20 when He said: unless your righteousness exceeds the righteousness of the scribes and Pharisees, you will by no means enter the kingdom of heaven. He’s now explaining what that exceeding righteousness looks like in order to further make His point that He is the only way to be saved.