In Matthew 21:44, Jesus says, "He who falls on this stone will be broken to pieces, but he on whom it falls will be crushed." The key to understanding this statement lies in the context of the verse and the larger conversation Jesus was having.
Jesus was teaching in the temple courts when the chief priests and elders approached Him and demanded to know the source of His authority. In response, Jesus asked them about John the Baptist—was he a prophet of God or not? The religious leaders, fearing the people’s response, refused to reveal their true opinion on the matter. In turn, Jesus refused to reveal the source of His authority (Matthew 21:23-27). In doing so, Jesus made it clear that the Jewish leaders themselves had no authority to judge Him.
Jesus then related two parables concerning vineyards. In the first, Jesus told of two sons who were told by their father to go work in the vineyard. The first son initially refused but later changed his mind and went to work. The second son promised to work, but he never went to the vineyard. Jesus applied this to the religious leaders of Israel, who were like the second son—they expressed agreement with the Father but, in the final analysis, were disobedient. The sinners who responded to John the Baptist’s message were like the first son—they seemed unlikely candidates for heaven, but they repented and thus will enter the kingdom (verses 28-32).
In the second parable, Jesus tells of a landowner who, at harvest time, sent some servants to his vineyard to collect the fruit. However, the farmers who were tending the vineyard were a wicked lot, and when the servants arrived, the farmers beat some of them and killed others. Finally, the landowner sent his own son to collect the fruit, expecting that the farmers would show him respect. But the farmers treated the son worst of all, throwing him out of the vineyard and killing him (Matthew 21:33-39).
Jesus then asks a question: "When the owner of the vineyard comes, what will he do to those tenants?" (Matthew 21:40). The chief priests and elders respond, "He will bring those wretches to a wretched end" (Matthew 21:41). Jesus then presses His point home with a quotation from Psalm 118: "The stone the builders rejected has become the capstone; the Lord has done this, and it is marvelous in our eyes" (Matthew 21:42). After a warning that the religious leaders will not inherit the kingdom (Matthew 21:43), we come to the statement in question, which is the culmination of a series of dire pronouncements aimed at the chief priests and elders.
Jesus begins with a question about John the Baptist in Matthew 21:25, but by the end of the conversation, Jesus is plainly speaking of Himself, referring to a "father" sending his "son" who was killed (Matthew 21:37). He then immediately quotes a Messianic prophecy (Matthew 21:42), in effect claiming to be the long-awaited Messiah. The progression is logical: a rejection of John leads one naturally to a rejection of Christ, to whom John pointed (John 1:29, 3:30).
The stone which "the builders rejected" in verse 42 is Jesus. Although rejected, He nevertheless becomes the "chief cornerstone" (NKJV). See also Acts 4:11; Ephesians 2:20; and 1 Peter 2:6-8. The builders’ rejection of the stone is a reference to Christ’s crucifixion. The Lord’s choice of the stone to be the cornerstone is a reference to Christ’s resurrection. God chose His Son, despised and rejected by the world, to be the foundation of His church (1 Corinthians 3:11). "See, I lay a stone in Zion, a tested stone, a precious cornerstone for a sure foundation" (Isaiah 28:16).
Now, there are consequences for coming into contact with a stone. If you trip over the edge of a rock and fall on it, you may break some bones. If a large enough rock falls on top of you, you may be killed. Jesus uses these truths to deliver a warning to the Jewish leaders.
The stone in verse 44 is also Jesus. In saying that those who fall on this stone "will be broken to pieces," Jesus is warning against opposing Him. Defying Jesus is like beating one’s head against a solid rock—a foolish action. In saying that those upon whom the stone falls "will be crushed," Jesus is warning against ignoring Him or trivializing Him. Apathy towards Jesus is like standing in the way of a falling rock—another foolish action. "I am here to do God’s work," Jesus essentially says. "The foundation for the church will be laid. It is unwise to oppose Me because God’s work is not inconsequential."
Rejection of the Savior is fatal. Unfortunately, many do reject Him. "He will be a stone that causes men to stumble and a rock that makes them fall" (Isaiah 8:14). To persistently reject the Savior is to court judgment so severe that the only thing left will be dust. The prophet Daniel gives a similar picture of the Messiah, likening Him to a rock "cut out, but not by human hands," which smashes into the nations of the world and completely obliterates them (Daniel 2:31-45).
Matthew 21:44 is a call to faith, an appeal to open one’s eyes and see that Jesus is indeed the Son of God sent into the world. The verse is also a strict warning against rejecting Jesus Christ. He is the sure Rock of salvation for those who believe, but an immovable stumbling stone for those who do not.