The Sermon and the Kingdom

Doing what Jesus said to do in the Sermon on the Mount is the faith that the risen Christ commanded His disciples to teach the nations.

Matthew 28:16-20
16 ¶ Then the eleven disciples went away into Galilee, into a mountain where Jesus had appointed them.
17 And when they saw him, they worshipped him: but some doubted.
18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth.
19 ¶ Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.

In the beginning of the Sermon on the Mount, Jesus declared:

Matthew 5:19-20
19 Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
20 For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.

The word "called" used in verse 19 means "named," as in how we talk about someone - thus, whoever shall break one of the least of the commandments of the Sermon, and shall teach others to break it as well, he shall be talked about as "the least" by those who are in the kingdom of heaven; it will be shown that he does not enter the kingdom. On the other hand, anyone who will both do what Jesus says to do in the Sermon, and teach others to do it too, shall be talked about as "great" by those who are in the kingdom of heaven.

Jesus repeats His warning as to how we should regard the Sermon: unless our righteousness, or obedience to the commandments of God, shall be more than the righteousness of the scribes and Pharisees, we shall never enter into the kingdom of heaven. The scribes and Pharisees broke the commandments of God, therefore they did not enter into the kingdom of heaven themselves, and they also stopped others from entering the kingdom of heaven by teaching them to break the commandments of God (Matthew 15:1-6; Matthew 23:13-15).

As Jesus tells us later in the Sermon:

Matthew 7:21-23
21 ¶ Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.

Among those who call Jesus "Lord," only those who do the will of Jesus's Father which is in heaven shall enter the kingdom of heaven. What is the will of Jesus's Father that we must do to enter the kingdom of Heaven? Jesus again tells us:

John 7:16-17
16 Jesus answered them, and said, My doctrine is not mine, but his that sent me.
17 If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself.

The word "doctrine" above means "teaching" or "instruction." That is what the Sermon is, as we see here:

Matthew 7:28-29
28 And it came to pass, when Jesus had ended these sayings, the people were astonished at his doctrine:
29 For he taught them as one having authority, and not as the scribes.

Now we know that the Sermon is not Jesus's doctrine, but His Father's - the Sermon is the will of the Father of Jesus. Therefore, among those who call Jesus "Lord," only those who do what Jesus said to do in the Sermon shall enter into the kingdom of heaven.

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Question: What is the common name for the set of all people who call Jesus "Lord"?
Answer: The common name for all those who call Jesus "Lord" is "Christianity." Jesus is talking about Christians.

Question: Christians who do not do what Jesus said to do in the Sermon on the Mount do not enter into the kingdom of heaven?
Answer: That's what He said.

Question: But doesn't that mean that Christians who do not do what Jesus said to do in the Sermon on the Mount cannot be saved in the kingdom of heaven?
Answer: How can one be saved in the kingdom of heaven if one shall not enter it?

Question: How is this compatible with John 3:16? "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life."
Answer: Jesus continued with this:

John 3:18-21
18 ¶ He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
19 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
20 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.
21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.

Those who hear and do what Jesus said to do in the Sermon are those that believe in Him.

Question: But what about grace? Surely we need not actually do what Jesus says and yet we shall enter into the kingdom of heaven, otherwise salvation is of works!
Answer:
"If ye love me, keep my commandments.

He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him.

If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.

He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father’s which sent me." (John 14:15, John 14:21, John 14:23, John 14:24)