St. Nikolai Velimirovich: "Before all else guard your heart" (Proverbs 4:23)
If you rule over the passions then you are a true king. Then your heart is in the hand of God. And this hand leads your heart without error to the kingdom of light and eternal life.
Where the physical circulatory system of blood is, there too is the spiritual circulatory system. The physical heart is an organ of the spiritual heart. And even if this is the reality, nonetheless this reality is a great mystery.
Truly this mystery is unfathomable for our mind, because the mind is smaller than the heart, and the heart comprises the mind, not the mind the heart. The part does not understand the whole, and neither do lower things understand higher things. God wanted it and arranged it this way. Blessed is he who searches the mysteries of God’s essence less and listens to God’s commandments more, and feels God’s love in his heart even more!
It is written in the Book of Life: The king’s heart is in the hand of the Lord (Proverbs 21:1). If you rule over the passions then you are a true king. Then your heart is in the hand of God. And this hand leads your heart without error to the kingdom of light and eternal life.
If in the first week of Great Lent you strengthened your conviction that your heart belongs to your God and Father; if you firmly decided to give it to Him, then in the second week learn to guard your heart. Engage in the valiant battle for the virginity and purity of your heart, so that you may return this gift to Him Who granted it to you.
Enlighten your heart with faith, strengthen it with hope, warm it with love, cense it with prayer, clean it with tears, feed it with the Blood of the Lord, and lift it up to the sky like a lit candle. Only in this way will you be able to peacefully await your passing from this earthly world to the heavenly world, without the terror of a sinner or an attack of conscience. Therefore, in the second week of Great Lent, frequently renew the Father’s guidance: Before all else guard your heart, for everything you do flows from it (Proverbs 4:23).
Excerpts from Sfantul Nicolae Velimirovici, Inima in Marele Post, Predania [The Heart in Great Lent: Sermons]
http://www.cuvantul-ortodox.ro/sfantul-nicolae-velimirovici-ganduri-pentru-postul-mare-a-doua-saptamana-pazeste-ti-inima/