Why do Christians try to put the Ten Commandments in public places and not the Beatitudes? The simple answer is that the Ten Commandments are for everyone, believer or unbeliever, and the Beatitudes are only for disciples, those who have been transferred into a covenant of grace by the Lord. It would be fruitless to put the beatitudes in a public place, because the world is unable to possess them. The beatitudes are descriptions of faithful Christians, and the only place where the unbelieving world factors in is in the latter part (verse 11), as people who insult and persecute faithful disciples.
The ten commandments have a use for the unbelieving world. They are the spelling out of the eternal conscience that all people have. All cultures know that they should not covet, steal, lie, murder, commit adultery, and disobey their parents. Everyone's conscience also tells them that there is a God and that we should love him, but many find successful ways to bury that truth. However, the ten commandments are the best, most universal way of getting across solid boundaries that prevent the world from harming their neighbors. All successful laws are founded on them, and even though the world will try to eradicate them from the public places, they cannot refute that they are the foundation of all good law.
The beatitudes, and the sermon on the mount in general, are a completely different animal. Only faithful disciples in a covenant of grace can even come close to following them. They run counter to the world's teaching, and whereas the world will look to the ten commandments and with honesty say that at least commandments 5-9 are rules to live by, the world teaches the opposite of the beatitudes. The sermon of the mount in its entirety contrasts the two covenants: the covenant of works that all are born into and the covenant of grace that only believers (true disciples) are transferred to by the Lord himself.
1. The beatitudes are descriptors of a faithful disciple.
2. True disciples are evangelists to the unbelieving world.
3. Following the ten commandments, even to the letter, does not make one a true disciple.
4. True disciples follow the spirit of the law, which is deeper than the letter of the law.
5. Social justice in the covenant of works does not look like social justice in the covenant of grace.
6. True disciples value heavenly things and not earthly things.
7. True disciples have peace, even in adversity.
8. True disciples do not judge others but help them to also become true disciples.
9. True disciples have a good relationship with God and their neighbors.
10. True disciples walk a narrow path impossible to walk for those in the covenant of works.
11. False teachers will teach the covenant of works as a rule for disciples and lead many astray.
12. Jesus concludes the sermon with an analogy of two foundations. The covenant of works is built on sand, while the covenant of grace is built on the rock of Christ.
The entirety of the law can be summarized thus: Love the Lord your God with all your heart, strength, soul, and mind; and love your neighbor as yourself. The ten commandments only touch the surface of this. Like I said before, the world does not keep the first four commandments at all, and they can't keep the last six, either from failure or indifference. A disciple who wants to keep them cannot do so in order to be saved. God saves us through his grace alone, by our faith alone, in Jesus Christ alone. The result of his salvation is the desire in the believer to please God and live in the covenant of grace as espoused in the sermon on the mount, not the ten commandments (although the sermon on the mount contains a deepened version of the ten commandments).
Let's look at loving your neighbor:
“Be careful not to practice your righteousness in front of others to be seen by them. If you do, you will have no reward from your Father in heaven.
“So when you give to the needy, do not announce it with trumpets, as the hypocrites do in the synagogues and on the streets, to be honored by others. Truly I tell you, they have received their reward in full. But when you give to the needy, do not let your left hand know what your right hand is doing, so that your giving may be in secret. Then your Father, who sees what is done in secret, will reward you. (Matthew 6:1-5)
How do I love the God whom I can't see? By loving my neighbor whom I can see. In the example above Jesus shows one in the covenant of works pretending to love his neighbor in order get praise by other people. This is not true love of neighbor. This is nowhere near love of God. Now look at John 21:
When they had finished eating, Jesus said to Simon Peter, “Simon son of John, do you love me more than these?”
“Yes, Lord,” he said, “you know that I love you.”
Jesus said, “Feed my lambs.”
Again Jesus said, “Simon son of John, do you love me?”
He answered, “Yes, Lord, you know that I love you.”
Jesus said, “Take care of my sheep.”
The third time he said to him, “Simon son of John, do you love me?”
Peter was hurt because Jesus asked him the third time, “Do you love me?” He said, “Lord, you know all things; you know that I love you.”
Jesus said, “Feed my sheep. (John 21:15-17)
Peter is in the covenant of grace. Jesus does not command him to do good works in order to gain God's love. God already loves him, and Christ has died for Peter's sins. Now Jesus asks Peter if he loves him. He asks him three times. Peter is not saying "yes" each time so that God will accept him. God has already changed his heart. He has already taken him from the covenant of works and put him into the covenant of grace. Peter's "yes" is in response to his trust in Christ for the forgiveness of his sins. He knows that God loves him. Does he love God? His answer is "yes." Now, how does he show that he loves God? Jesus tells him: "feed my sheep." Love your neighbor, not to be seen by others, not to be rewarded, but to love the God whom you cannot see.
Peter himself affirms this in his first epistle: "Shepherd the flock of God among you, exercising oversight NOT UNDER COMPULSION." In other words: not from within a covenant of works. Not out of fear of hell or reward of heaven but out of love for God. Not under compulsion, Peter tells us, "but voluntary, according to the will of God." Yes, God wills his believers to shepherd the flock of God, because they are now operating within a covenant of grace. In God's grace we are subject to his will alone. Peter finishes, "not for sordid gain (Matthew 6:1-5), but with eagerness (John 21:15-17)."
You don't have to be a pastor to shepherd God's flock. True disciples share the gospel with each other and study the bible together. They seek Christ and his kingdom together. And they do all things for each other out of love for God. They do not seek to be praised by others. They don't do it because they feel they have to or God will punish them. They offer up their lives each day to serve their brothers and sisters--whom God has given them to serve--because loving their neighbors is loving the Lord who has eternally saved them. God brings them from being rebellious self-servers into faithful God-pleasers. God loves you. Do you believe that? Are you loving the brethren?