These two brief verses broadly cover conduct, motivation, attitude, and prioritizing one's life. To meet these qualifications requires "truth in the inward parts" (Psalm 51:6). A deceitful heart will never meet the standards because it does not operate from a foundation of godly integrity. David says in verse 5 that, humanly, he was shaped in iniquity. God, with our cooperation through faith, is ultimately the Creator of a pure heart in us, but it is a protracted process achieved by imparting a holy nature by His Spirit. This unites us with a holy Christ, with whom we fellowship, washing us in the blood of the Lamb so that with His aid we can mortify the flesh and live toward God, giving Him first priority in everything.
John W. Ritenbaugh
The Beatitudes, Part 6: The Pure in Heart