If you are like me, you often hear the word Gospel and think we must be talking about God’s holiness, man’s sinfulness, the cross, Jesus resurrection, justification by faith alone, or repentance. Then you read the Sermon on the Mount and it is possible miss it because we have become geared to hear the Gospel only with certain key terms.
The Gospel is in the Sermon on the Mount. Here is the Gospel,
“Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill” (Matthew 5:17).
When Jesus says He came to fulfill, what does He mean? First, the word fulfill here seems to communicate the idea “complete.”
Jesus completes both the Law and the Prophets. He did not just fulfill the Law, but also the prophets. RT France summarizes this well, “His life and ministry has brought that to which they pointed forward. Is it possible to understand his fulfilling of the law in the same light?” Yes, the Law (or Torah) is a tutor leading us to Christ (Gal 3:24).
France again says, “The Torah, then, is not God’s last word to his people, but is in a sense provisional, looking forward to a time of fulfillment through the Messiah.” The same commentator summarizes the idea. “Far from wanting to set aside the law and the prophets, it is my [Jesus] role to bring into being that to which they have pointed forward, to carry them into a new era of fulfillment.” 
The pointing forward to a future completion is further expanded on and confirmed in the next verse, “For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished” (Matt 5:18).
The significance of the Law will never pass away. Every part of the Law, down to the smallest detail is important, but there will be a time when it completes its goal. “The law, down to its smallest details, is as permanent as heaven and earth, and will never lose its significance; on the contrary, all that it points forward to will in fact become reality.” 
But, this begs the question, what does the Law point forward to requiring Jesus to complete or fulfill? Answer, death. He is the perfect, blameless, lamb of God who takes away the sins of the world by dying on the cross (or the altar in heaven).
The Law defines sin and restoration. Atonement happened through the sacrifices of bulls, goats, and other sacrifices (read almost all of Leviticus). The problem with Law sacrifices is their insufficiency to perfect believers. “For it is impossible for the blood of bulls and goats to take away sins” (Heb 10:4). Yet blood must be shed for sins to be forgiven. How do we learn this necessity? From the Law. “And according to the Law, one may almost say, all things are cleansed with blood, and without shedding of blood there is no forgiveness” (Heb 9:22).
So the Law teaches us forgiveness needs blood to be shed and yet the offerings in the Law are insufficient to atone for our sins. But Christ, being the blameless, sinless, High Priest, is the sacrifice on our behalf so that we may be sanctified and perfected. “By this will we have been sanctified through the offering of the body of Jesus Christ once for all” (Heb 10:10) and “For by one offering He has perfected for all time those who are sanctified” (10:14).
So how does Jesus complete the Law? He does so by dying on the cross as God’s perfect, blameless sacrifice — the Lamb of God. When we understand the Law’s function and what it points forward too, then we can understand how Jesus gives us the Gospel in the Sermon on the Mount. Jesus told His people about Easter before Easter.
 The Gospel of Matthew NICNT, 183.
 RT France, The Gospel of Matthew, NICNT, 184.