Many of us are familiar with the "Blessed are..." portions of Jesus' sermon on the mount and sadly, some think that's actually the entire sermon. But it's not. Our Lord's sermon spans two chapters in the Apostle Matthew's gospel, with the major theme being: live holy lives unto the Lord. Though the Beatitudes are very important, it's only the short introduction to Christ's great teaching on what a true Christian's life looks like.
The Lord Jesus strongly teaches that our mere profession of faith doesn't prove we belong to Him. Only the demonstration of the Holy Spirit's work gives evidence that we've been sealed (2 Cor 1:22). But if the fruit of righteousness isn't born in all who profess the name of Christ, it's clear they're not possessed by the life of Christ.
Sermon on the mount in a nutshell:
1) Blessed are those who are in Christ and live as He lived (Mt 5:1-12)
2) I should be physical evidence (the taste) that the LORD is good; He alone preserves life (Mt 5:13)
3) My life should be a beacon of the Gospel, not a cavern of megalomania (Mt 5:14-16)
4) Obey God's commandments (Mt 5:17-20)
5) Quickly settle disagreements with love (Mt 5:21-26)
6) Don't be sexually immoral in heart, mind or body (Mt 5:27-30)
7) MEN, don't divorce your wives (Mt 5:31-32)
8) Don't lie or flatter; be a person of my word; be trustworthy (Mt 5:33-37)
9) Be humble and loving, not selfish and vengeful (Mt 5:38-42)
10) Be perfect in Christ and love righteousness (Mt 5:43-48)
11) Give to others as unto the Lord, not for praise (Mt 6:1-4)
12) Pray in accordance with God's will not mine or others (Mt 6:5-13)
13) Humbled and grateful for God's mercy, forgive others as God has forgiven me (Mt 6:14-15)
14) Practice holy fasts to the Lord, not to self-righteous asceticism (Mt 6:16-18)
15) Invest my life in Christ rather than in my body and life on earth (Mt 6:19-21)
16) Don't watch or read things that cause me to sin (Mt 6:22-23)
17) Live joyfully abandoned to Christ, not foolishly in love with this world (Mt 6:24)
18) God is omniscient, sovereign and good and always provides for His own (Mt 6:25-34)
19) Judge in accordance with God's word, not with my opinions or personal affinities (Mt 7:1-5)
20) Be wise who I invest in; true disciples, not flatterers (Mt 7:6)
21) Trust God to provide all good things (Mt 7:7-11)
22) Do the good to others I'd like done to me, whether or not it's done to me (Mt 7:12)
23) Be in the minority; persevere in holiness rather than self-gratification (Mt 7:13-14)
24) Discern between true disciples of Christ and false converts (Mt 7:15-20)
25) Christ will reject many who profess faith, but haven't truly been regenerated (Mt 7:21-23)
26) Only those who persevere in the will of God prove to be His (Mt. 7:24-27)
27) Jesus spoke with authority as God and not as a human theologian (Mt 7:28-29)
If the Son of God did not disrobe Himself of His glory to put on sackcloth and ashes (skin and bones), descend from Heaven to fulfill the requirements of God's righteousness, take the blame and shame for our sins, suffer, die and resurrect anew, then all of us would be without hope of ever attaining eternal life. But because all this did happen and Christ did come and lived a sinless life, bore the just judgement for our sins, suffered, died, was buried, rose and ascended back to Heaven, having completed His work, now seated at the right hand of the Father; all who believe and call on the name of the Lord, will be saved.
However, not all who call on the name of the Lord, actually believe the Lord. This is the message of the sermon on the mount: saved people live like Christ, not for a time, but for a lifetime. False converts live in opposition to what they profess, living how they really believe—that they are their own god and live in accordance with what they feel is right instead of what God says is right.
Because of the sermon on the mount, I am deeply grieved for the many (that Jesus says) will be shocked to find out they are not His. On the day of Judgement, they will discover all their "righteous" deeds were only filthy rags in God's sight, because they weren't done in faith and by the power of God. But they were performed for various self-satisfying reasons and for the praise of others (i.e., "I did that because it made me feel good" rather than "I did that because it pleases God").
A true Christian feels good because they do what is pleasing to God (as clearly revealed in His Word), rather than thinking because they feel good, God must be pleased.
"Every way of a man is right in his own eyes, but the LORD weighs the heart. To do righteousness and justice is more acceptable to the LORD than sacrifice."
~Proverbs 21:2-3 (ESV)