“Blessed are the pure in heart, for they shall see God” (Matthew 5:8).
Purity in heart is the sixth key to living a blessed life. Jesus draws our attention immediately to the internal workings of our own hearts, and not to the pious appearances we might display in the public realm. God sees what is really going on and points us to the matter of primary importance; “People look at the outward appearance, but the Lord looks at the heart” (1 Samuel 16:7).
But hang on. What sort of “purity” in the heart is Jesus referring to here in this beatitude? I don’t get the impression that Jesus is teaching that purity of heart means achieving sinless perfection. The apostle John makes it abundantly clear that those who claim to have no sin are deceiving themselves and the truth is not in them (1 John 1:8). So then, what does it mean to have a pure heart? James 4:8 gives us some insight, when James calls upon the Jewish Christians of the Dispersion:
“Come near to God and he will come near to you. Wash your hands, you sinners, and purify your hearts, you double-minded.”
The men who James describes here as ‘double-minded’ are called to purify their hearts. These are men whose hearts are torn in their commitments between the Lord and the world, and James here exhorts them to draw ‘near to God’ and reestablish their allegiance to Him. Jesus even affirms this idea of purity in the Gospels: “Love the Lord your God with all your heart…” (Matthew 22:37). Sincerity and undivided attention to God and His purposes are the evidence of a pure heart. Those who possess pure hearts align themselves single-mindedly to the magnification of God’s worth with all that they are and with all that they do. And what will the reward be for those who offer their lives as living sacrifices to the King of all creation?
They will see Him. They will see the final manifestation of God’s glory when Jesus the King establishes his kingdom in full on the day of reckoning. They will be admitted into his presence to enjoy sweet communion with Him. And once more it is worth pointing out that though this blessing finds its fulfilment in the future, there are still varying degrees to which those with pure hearts can still experience the foretaste of this blessing in the present. The pure in heart who still take residence here in this broken world see glimpses of the display of God’s radiance in the present state of His kingdom. The pure in heart also enjoy God’s presence through admission to His chosen church community, as believers join together in worship before the Lord.
Piper says it well when he says, “Purity is a prerequisite for seeing God.” 
An uncompromising desire to please God in all things is one of the trademarks of a holy person who is fit to see the King of glory (cf. Hebrews 12:14). But who here on this sin-saturated earth is fit to see the Most High King? Who here is capable of withstanding exposure to His holy radiance and still stand on their two feet unscathed?
Who on earth can say, “I have kept my heart pure; I am clean and without sin?” (Proverbs 20:9). Once again, we are faced with a predicament that only the Lord can solve. Though it may be impossible for us to do a detailed cleansing of our spiritual hearts, “with God all things are possible” (Matthew 19:26). So let’s first seek God’s grace, asking Him to do the work of purification in our hearts by praying along with King David from Psalm 51:10,
“Create in me a pure heart, O God, and renew a steadfast spirit within me.”
 – Piper, J. (2020). Blessed Are the Pure in Heart. [online] Desiring God. Available at: https://www.desiringgod.org/messages/blessed-are-the-pure-in-heart [Accessed 19 Feb. 2020].