1 John 3:19

By this we know that we are of the truth, and before Him we shall assure our hearts. For if our heart condemns us, God is greater than our heart, and knows all things. "By this we know." By what? From the fact that we …

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 meaneth, "before Him?" Where He seeth. Whence the Lord Himself in the Gospel saith: "Take heed that ye do not your righteousness before men, to be seen of them: otherwise ye have no reward with your Father which is in heaven." And what meaneth, "Let not thy left hand know what thy right hand doeth:" 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 worketh, 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 aught that is good. And where do we get to know this? Thou art before God: question thine heart, see what thou hast done, and what therein was thine aim; thy salvation, or the windy praise of men.

On hearing these words, I wept most violently. When he saw me weeping, he said to me, "Why do you weep?" And I said, "Because, sir, I know not if I can be saved." "Why?" said he. And I said, "Because, sir, I never spake a true word in my life, but have ever spoken cunningly to all, and have affirmed a lie for the truth to all; and no one ever contradicted me, but credit was given to my word. How then can I live, since I have acted thus?"

He had said, "Let us not love only in word and in tongue, but in work and in truth:" we are asked, In what work, or in what truth, is he known that loveth God, or loveth his brother? Above he had said up to what point charity is perfected: what the Lord saith in the Gospel, "Greater love than this hath no man, that one lay down his life for his friends," this same had the apostle also said: "As He laid down His life for us, we ought also to lay down our lives for the brethren." This is the perfection of charity, and greater can not at all be found. But because it is not perfect in all, and that man ought not to despair in whom it is not perfect, if that be already born which may be perfected: and of course if born, it must be nourished, and by certain nourishments of its own must be brought unto its proper perfection.

In this we know that we are of the truth. That is, when we perform acts of piety in truth, it is evident that we are of the truth, which is God, just as we imitate His perfection according to our capacity.