Jamieson, Fausset and Brown on 2 Corinthians 3:2

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."