3 This it is then that he enforces here. “In this we know that we are of the truth, when in deed and in truth” we love, “not only in words and in tongue: and assure our heart before Him.” What means, “before Him?” Where He sees. Whence the Lord Himself in the Gospel says: “Take heed that you do not your righteousness before men, to be seen of them: otherwise you have no reward with your Father which is in heaven.” And what means, “Let not your left hand know what your right hand does:” except that the right hand means a pure conscience, the left hand the lust of the world? Many through lust of the world do many wonderful things: the left hand works, not the right. The right hand ought to work, and without knowledge of the left hand, so that lust of the world may not even mix itself therewith when by love we work anything that is good. And where do we get to know this? You are before God: question your heart, see what you have done, and what therein was your aim; your salvation, or the windy praise of men. Look within, for man cannot judge whom he cannot see. If “we assure our heart,” let it be “before Him.” Because “if our heart think ill of us,” i.e. accuse us within, that we do not the thing with that mind it ought to be done withal, “greater is God than our heart, and knows all things.” Thou hidest your heart from man: hide it from God if you can. How shall you hide it from Him, to whom it is said by a sinner, fearing and confessing, “Whither shall I go from Your Spirit? And from Your face whither shall I flee?” He sought a way to flee, to escape the judgment of God, and found none. For where is God not? “If I shall ascend,” says he, “into heaven, You are there: if I shall descend into hell, You are there.” Whither will you go? Whither will you flee? Will you hear counsel? If you would flee from Him, flee to Him. Flee to Him by confessing, not from Him by hiding: hide you can not, but confess you can. Say unto Him, “You are my place to flee unto;” and let love be nourished in you, which alone leads unto life. Let your conscience bear you witness that your love is of God. If it be of God, do not wish to display it before men; because neither men's praises lift you unto heaven, nor their censures put you down from thence. Let Him see, who crowns you: be He your witness, by whom as judge you are crowned. “Greater is God than our heart, and knows all things.”
4. “Beloved, if our heart think not ill of us, we have confidence towards God:” — What means, “If our heart think not ill”? If it make true answer to us, that we love and that there is genuine love in us: not feigned but sincere; seeking a brother's salvation, expecting no emolument from a brother, but only his salvation— “we have confidence toward God: and whatsoever we ask, we shall receive of Him, because we keep His commandments.” — Therefore, not in the sight of men, but where God Himself sees, in the heart— “we have confidence,” then, “towards God: and whatsoever we ask, we shall receive of Him:” howbeit, because we keep His commandments. What are “His commandments”? Must we be always repeating? “A new commandment give I unto you, that you love one another.” It is charity itself that he speaks of, it is this that he enforces. Whoso then shall have brotherly charity, and have it before God, where God sees, and his heart being interrogated under righteous examination make him none other answer than that the genuine root of charity is there for good fruits to come from; that man has confidence with God, and whatsoever he shall ask, he shall receive of Him, because he keeps His commandments.
Source: Homilies on the First Epistle of John (New Advent)