When I was growing up, the "Sermon on the Mount", recorded in Matthew chapters 5 through 7 was considered to be something of a manifesto of the Christian life, an outline of how the disciples of Jesus ought to live and walk as we follow Him. This discourse from the mouth of the Lord Jesus includes the well-known "beatitudes", the "salt and light" teaching, His revelations about adultery and divorce, how to give alms, how to pray and fast, the narrow vs the wide gate, and the wise man who built his house upon a rock, among other very familiar and beloved teachings!
It is troubling to learn now, as an adult, that some Christians believe and teach that this passage was not even written to the majority of the body of Christ. They say it was not written to "gentile" believers, but Jewish. What?? I'm afraid that modern Christians have gotten so full of religious ideas that they've forgotten the fine art of listening to the Shepherd, and obeying His voice.
I still believe that the sermon of Jesus recorded in Matthew 5-7 is just as much ours as the Great Commission recorded in chapter 28 is. He said:
"Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you..."
Does anyone doubt that the Great Commission is given to the entire body of believers, even today? Is it not the responsibility of the gentile church to go to the nations to teach them the good news of the gospel, baptize them, and teach/disciple them in "all things whatsoever I [Jesus] have commanded you"? Two important things jump out at me.
1) The people present when He gave the "Great Commission" were also all Jewish Believers, just as were those who heard Him on the Mount of Olives. Would these modern believers who deny the Sermon on the Mount was written to us, also deny the Great Commission was written to us? Same audience. (Matthew 5:1-2; and 28:16) Also...
2) Part of the Great Commission states that we are to teach new believers all the things Jesus commanded. That would include all the things given at the Sermon on the Mount. Correct?
Now, here is the problem some modern believers have with the Sermon on the Mount, and why they insist that it was written specifically to Jewish believers, excluding gentile believers. They do not want all of it to apply to them. They especially do not like the part where Jesus said,
17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.
18 For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
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.