Ye are our epistle written in our hearts, known and read of all men:

People's New Testament

3:2 Ye are our epistle. The church itself owed its existence to him. He could point to his work, to the disciples, as his letter of commendation. He was known by his fruits.

Written in our hearts. When he looked into his heart, he saw them enshrined there, and felt that he needed no commendation to them.

Wesley's Notes

3:2 Ye are our recommendatory letter - More convincing than bare words could be. Written on our hearts - Deeply engraven there, and plainly legible to all around us.

Jamieson-Fausset-Brown Bible Commentary

2. our epistle-of recommendation.

in our hearts-not letters borne merely in the hands. Your conversion through my instrumentality, and your faith which is "known of all men" by widespread report (1Co 1:4-7), and which is written by memory and affection on my inmost heart and is borne about wherever I go, is my letter of recommendation (1Co 9:2).

known and read-words akin in root, sound, and sense (so 2Co 1:13). "Ye are known to be my converts by general knowledge: then ye are known more particularly by your reflecting my doctrine in your Christian life." The handwriting is first "known," then the Epistle is "read" [Grotius] (2Co 4:2; 1Co 14:25). There is not so powerful a sermon in the world, as a consistent Christian life. The eye of the world takes in more than the ear. Christians' lives are the only religious books the world reads. Ignatius [Epistle to the Ephesians, 10] writes, "Give unbelievers the chance of believing through you. Consider yourselves employed by God; your lives the form of language in which He addresses them. Be mild when they are angry, humble when they are haughty; to their blasphemy oppose prayer without ceasing; to their inconsistency, a steadfast adherence to your faith."

Matthew Henry's Concise Commentary

3:1-11 Even the appearance of self-praise and courting human applause, is painful to the humble and spiritual mind. Nothing is more delightful to faithful ministers, or more to their praise, than the success of their ministry, as shown in the spirits and lives of those among whom they labour. The law of Christ was written in their hearts, and the love of Christ shed abroad there. Nor was it written in tables of stone, as the law of God given to Moses, but on the fleshy (not fleshly, as fleshliness denotes sensuality) tables of the heart, Eze 36:26. Their hearts were humbled and softened to receive this impression, by the new-creating power of the Holy Spirit. He ascribes all the glory to God. And remember, as our whole dependence is upon the Lord, so the whole glory belongs to him alone. The letter killeth: the letter of the law is the ministration of death; and if we rest only in the letter of the gospel, we shall not be the better for so doing: but the Holy Spirit gives life spiritual, and life eternal. The Old Testament dispensation was the ministration of death, but the New Testament of life. The law made known sin, and the wrath and curse of God; it showed us a God above us, and a God against us; but the gospel makes known grace, and Emmanuel, God with us. Therein the righteousness of God by faith is revealed; and this shows us that the just shall live by his faith; this makes known the grace and mercy of God through Jesus Christ, for obtaining the forgiveness of sins and eternal life. The gospel so much exceeds the law in glory, that it eclipses the glory of the legal dispensation. But even the New Testament will be a killing letter, if shown as a mere system or form, and without dependence on God the Holy Spirit, to give it a quickening power.