Plenty of Christ’s statements in the Beatitudes—and throughout the Sermon on the Mount—drew the ire and indignation of the Jewish religious leaders, along with anyone else committed to their system of works-righteousness. And perhaps nothing was as direct an assault on their legalism as His words in Matthew 5:8, “Blessed are the pure in heart, for they shall see God.”
John MacArthur calls it “one of the greatest utterances in all the Bible. It stretches over everything else revealed in Scripture.”  John MacArthur, The Only Way to Happiness (Chicago: Moody Publishers, 1998), 147. And while the full significance of this Beatitude might not be self-apparent to our twenty-first century eyes, a little investigation will help us appreciate all Christ’s words meant to His original audience.
The Burden of the Law
Put yourself in the sandals of an average Jewish man or woman during Jesus’ life and ministry. The Pharisees and other religious leaders had concocted an oppressive system of laws and traditions that governed virtually every aspect of daily life. As John Macarthur explains,
The Pharisees had misinterpreted the Law of Moses. They had invented new laws so that, if they couldn’t keep God’s laws, they could pacify their consciences by keeping traditions. The people found it impossible to perform as required under this rigid legalistic system. In fact, the leaders themselves had decided that if one could keep just a few of the laws, God would understand. Then they couldn’t even do that, so they agreed that if one could just find one law and keep it, God would understand. (Hence the question of the lawyer in Matthew 22:36.)
So the mass of people in Israel were frustrated by a legal system they could not keep, and it produced in them tremendous guilt and anxiety. The Only Way to Happiness, 148-149.
The Futility of the Law
Worse still, it was a system that had no power to save anyone in the first place. Collectively, Israel longed to know the forgiveness David described in Psalm 32:2, “How blessed is the man to whom the Lord does not impute iniquity”—a forgiveness they could not achieve through adherence to the Law. As John MacArthur explains,
There are only two kinds of religion in the world. Only two. One is the religion of human achievement, which comes under every brand imaginable but is all from the same base; namely, you earn your own way. The other is the religion of divine accomplishment that says, “I can’t do it. God did it in Christ.” The Only Way to Happiness, 155.
While Israel could not yet point to the completed work of Christ, they had forgotten the words of 1 Samuel 16:7, “For God sees not as man sees, for man looks at the outward appearance, but the Lord looks at the heart.” No matter how pious they were on the outside, the rigid laws and traditions could do nothing to cleanse their hearts.
If they were honest with themselves, even the religious elite understood that the Law was not enough to save them. Nicodemus was “the teacher of Israel” (John 3:10, emphasis added), yet he knew that he required a righteousness the Law could not provide.
Everywhere Christ went, the question was the same: “What shall I do to inherit eternal life?” (Luke 10:25). In this simple Beatitude, Christ is saying that eternal life is not available through anything you can do.
The purity of heart Christ referenced in Matthew 5:8 is the result of God’s work, not man’s. This is a work the Lord promised to do long ago.
Then I will sprinkle clean water on you, and you will be clean; I will cleanse you from all your filthiness and from all your idols. Moreover, I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. (Ezekiel 36:25-26)
Rather than pinning their hopes on their own good works, the Jews should have been looking ahead to the Redeemer promised in Isaiah’s prophecy: The One who would bear their griefs and sorrows; who would be “pierced through for [their] transgressions” and “crushed for [their] iniquities;” and on whom the Lord would cast the full burden of all their sins (Isaiah 53:4-6).
A pure heart is only available through the work of Christ. In 2 Corinthians 5:21, Paul says it as plainly as it can be said, “He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him.”
On the cross, God assigned to Christ the penalty of all your sins, so that He could assign the rich rewards of Christ’s righteousness to you. There’s nothing you and I can do to purify our own hearts. Paul identified the source of his righteousness this way: “not having a righteousness of my own derived from the Law, but that which is through faith in Christ, the righteousness which comes from God on the basis of faith” (Philippians 3:9).
The imputation of a pure heart is pictured in the language Christ used in Matthew 5:8.
Katharos [pure] is a noun form from katharidzo, which means to cleanse from filth and iniquity. It means to be free from sin. It is akin to the Latin word castus, root of the English word chaste. Medical people know that a cathartic is an agent used to cleanse a wound or infected area in order to make it pure. The Only Way to Happiness, 160.
Only through the work of Christ can our hearts be purified. “For as through the one man’s disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous” (Romans 5:19).
In Matthew 5:8, Christ says that only the pure in heart “shall see God.” In his commentary on Matthew’s gospel, John MacArthur describes the glorious riches of that blessed promise.
When our hearts are purified at salvation we begin to live in the presence of God. We begin to see and to comprehend Him with our new spiritual eyes. Like Moses, who saw God’s glory and asked to see more (Exodus 33:18), the one who is purified by Jesus Christ sees again and again the glory of God.
To see God was the greatest hope of Old Testament saints. Like Moses, David wanted to see more of God. “As the deer pants for the water brooks,” he said, “so my soul pants for Thee, O God. My soul thirsts for God, for the living God; when shall I come and appear before God?” (Psalm 42:1). Job rejoiced when he was able to say, “I have heard of Thee by the hearing of the ear; but now my eye sees Thee” (Job 42:5).  John MacArthur, The MacArthur New Testament Commentary: Matthew 1-7 (Chicago: Moody Press, 1985), 207-208.
Of course, Job hadn’t yet seen God with his physical eyes. But through eyes of faith he had grasped God’s greatness and majesty. He could finally appreciate—as we can and should—God’s sovereignty, His love, His grace, and His mercy. Think of it as a foretaste in this life of the great joys the apostle John described: “Beloved, now we are children of God, and it has not appeared as yet what we will be. We know that when He appears, we will be like Him, because we will see Him just as He is” (1 John 3:2).
Because Christ has purified our hearts, we will one day stand in His presence and see Him face-to-face. What greater blessing could there be?