Purity of heart is a human need because the human heart is caught by various desires. Some more superficial. Others are deeper. Some are temporary, and others last longer. Some desires align with what will give us true happiness, and others are misleading. Saint Augustine writes: Many are the desires of the human heart, but the will of the Lord remains eternally (De Trinitate 15.38). The eternal desire of God is us! We all long for uninterrupted happiness, but tragically not all of us desire faith (De Trinity 4.6).
Therefore blessed is the person who manages to discern in her heart the primordial desire of God. Either to serve God, or to be with God. Or how about both! Think of the sisters of Lazarus, Mary and Martha (Luke 10:38-42). One was preparing the meal and the other one was listening to Jesus. I think we have a little of each one of those sisters within us. They are sisters because to serve God is to be present in the world for Him and his wishes, which is a sister act of waiting for him in stillness of heart. God is pleased with those who await mercy from Him (Psalm 117) and God is pleased with those who follow His commandments.
The responsorial Psalm used in liturgy on the feast of Saint Augustine and also at other days during the year says: One thing I ask of the Lord: to dwell in the House of the Lord and to contemplate the delights of the Lord (Psalm 27:4). What would it be to contemplate the delights of the Lord, if not delights that never end and whose intensity always increases? The House points in the thought of Augustine to Heaven where we will be delighted and filled with joy. Nothing in heaven could take that joy away from us, since on this earth joys are always vulnerable to ending with the arrival of bad and very serious news that could reach us at any time, but that possibility will not happen in heaven, because in heaven, what we see on earth as bad news will be seen in the light of God’s countenance, which will not just reassure us, but will not allow doubt in our hearts. God will allow us to see beyond the limitations of our current state, and the cause of our joy will be infinitely great, namely God himself.
In the Beatitudes, Jesus says that to reach the vision of God, a pure heart is needed. What purifies the heart? According to Saint Augustine in his work On The Trinity, faith and love purify the heart. He writes: Vision is the prize of faith, and it is faith that purifies hearts and makes this reward attainable, as it is written: Purifying his hearts by faith (Acts 15: 9) (De Trinitate 1.17). Being purified by faith means always living in the truth: since faith transcends reason to lead us to revealed truth. Faith is like the light that removes from us the darkness of our intellect, and also of cloudy emotions, and allows us to see the truth: It is written: God is light; but do not think that it is this light that the eyes contemplate, but a light that the heart intuits when you hear it said: God is true (1 Jn 1:5) (De Trinitate 8.3).
In addition to faith, love is also necessary. It has been said that loving our neighbor is the other side of the coin of loving God, and in order to truly love, due to the weakness and fallibility of our love, we need to receive a greater force, namely the very force of the Holy Spirit: Let us love Him, then, and let us unite to Him through the love that is spread in our hearts by the Holy Spirit, who has been given to us (Rom 5:5; De Trinitate 7.5) Love is seen in good works: to achieve purity, we are required to act righteously. (De Trinitate 1.31) Acting righteously is nothing more than loving your neighbor as yourself. In this way our neighbor, and in fact all of creation, help us to prepare our hearts to be able to contemplate God: it is necessary to purify the heart by making good use of all the realities that the eyes see and the ears perceive. (De Trinity 3.26).