Specifically, at various times and places, and from within certain faith traditions, three problems have been noted about the Sermon on the Mount.
1. Practically Impossible
Jesus' demands in the Sermon seem so high that the Sermon appears to be practically impossible to obey. Consider:
But I say to you that everyone who is angry with his brother will be liable to judgment.No anger. No lust. Non-retaliation in the face of physical assault. Loving our enemies. Most people don't think they can obey these commands. At least not with any consistency and regularity. If you try to obey the Sermon you'll live a life of chronic failure.
I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.
Do not resist the one who is evil. But if anyone slaps you on the right cheek, turn to him the other also.
I say to you, Love your enemies and pray for those who persecute you.
But maybe that's exactly the point, theologians like Martin Luther have argued. Maybe the Sermon was intentionally made to be practically impossible in order to humiliate and expose any attempts at works-based righteousness.
I don't care if you agree with Luther or not, his teachings about the Sermon illustrate my point: we object to the Sermon because we think it's practically impossible. It just can't be obeyed.
2. Theologically Problematic
We also object to the Sermon because we find it to be theologically problematic. Specifically, Jesus seems to be a legalist. Even worse, there is no atonement theology in the Sermon. Salvation in the Sermon is earned through obedience. Consider:
For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven.3. Politically Irresponsible and Immoral
If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell.
You therefore must be perfect, as your heavenly Father is perfect.
If you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses.
Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you.
Finally, it is argued by any Christian who is not a pacifist that the Sermon is just not politically responsible. Many point out that Jesus' call to non-violence in the Sermon could never be the ethic of nation states.
Further, it is argued, in the face of evil Christians must resort to violence. To obey the command "do not resist an evil person" isn't just hard to do, it's immoral.
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And yet, all these objections are eternally haunted by Jesus' final words and warning:
"Everyone then who hears these words of mine and does them will be like a wise man who built his house on the rock. And the rain fell, and the floods came, and the winds blew and beat on that house, but it did not fall, because it had been founded on the rock.
And everyone who hears these words of mine and does not do them will be like a foolish man who built his house on the sand. And the rain fell, and the floods came, and the winds blew and beat against that house, and it fell, and great was the fall of it.”