Lloyd-Jones, in Studies in the Sermon on the Mount:
What, then, are the characteristics of the true Christian? Put positively, it is that he “does the will of my Father who is in heaven.” Our Lord says: “Not every one that says to me, ‘Lord, Lord’ will enter the kingdom of heaven, but he that does the will of my Father who is in heaven.” “Therefore whoever hears these words of mine, and does them, I will compare him to a wise man.” What does this mean?
The first part of the answer is to make clear what it does not mean. This is most important. Obviously it does not mean justification by works. Our Lord is not saying here that the man who is truly a Christian is the man who, having listened to the Sermon on the Mount, puts it into practice and thereby makes himself a Christian.
Why is that interpretation impossible? For the good reason that the Beatitudes make it quite impossible. At the very beginning we emphasized that the Sermon on the Mount must be taken as a whole, and so it must. We start with the Beatitudes, and the first statement is: “Blessed are the poor in spirit.” We can try from now until we are dead, but we shall never make ourselves “poor in spirit,” and we can never make ourselves conform to any of the Beatitudes. That is a sheer impossibility, so it cannot mean justification by works.
Then take the great climax at the end of the fifth chapter: “You must be perfect, as your heavenly Father is perfect.” That again is quite impossible to man in his own strength, and proves further that this passage does not teach justification by works.
Were it to do so it would contradict the whole message of the New Testament, which tells us that what we have failed to do, God has sent His Son into the world to do for us — “with men this is impossible, but with God all things are possible.” No man shall be justified by the deeds of the law [including obeying the Sermon on the Mount], but only by the righteousness of Jesus Christ.
That is a critical point: It would be an utter abuse and misunderstanding of the Sermon on the Mount to think that we become Christians or right with God by putting it into practice. Instead, you cannot put it into practice unless you have first been set right with God apart from your works and adopted into his family.
This is not only the teaching of the NT in general (Romans 3:19-20;8:3-4; Ephesians 2:8-10; Titus 3:5; etc.), but is also made clear right at the entry of the Sermon on the Mount. For to be “poor in spirit” (Matthew 5:3) is to despair of your own efforts in being accepted by God and to look to Christ only for your acceptance before God. This is also part of the meaning of being one who “hungers and thirsts for righteousness” (5:6). Notice also how Jesus is talking to people who have God as “Father” throughout the sermon (6:4; 6:6; 6:32) — that is, he is addressing Christians.
Don’t Pull a Jim Marshall
Jim Marshall was a defensive player for the Vikings in the 60s who is known for his famous “wrong way run.” In a game against the San Francisco 49ers, he recovered a fumble and returned it 66 yards the wrong way into his own end zone. Thinking he had scored a touchdown, he then threw the ball in celebration, resulting in a safety for the 49ers. This is considered by many to be “one of the most embarrassing moments in professional sports history.” (The video is below.)
If we try to obey the Sermon on the Mount as a way of seeking acceptance with God, we are pulling a Jim Marshall. We are doing the exact opposite of what the Sermon on the Mount actually requires, and what God requires. Such “obedience” does not really count, because it is not actually obedience. It might look like it in a sense — just like Jim Marshall’s run looks almost exactly like an ordinary touchdown. But it’s actually going the wrong way, and in the end not only doesn’t count, but scores for the other team.
Here’s the video of Jim Marshall’s run:
One last thing: This is not the end of the story. The Vikings ended up winning the game 27-22 on a touchdown that they scored off a fumble that Marshall caused when sacking the 49ers quarterback. I say this here because I don’t want to give the impression that I’m picking on Jim Marshall here! But it’s also a fitting completion to the analogy, because it means if we have sought acceptance with God by our obedience, we aren’t stuck. We can repent and trust him for acceptance apart from works so that then, in turn, we can truly do the good works commanded in the Sermon on the Mount.