Notes on The Holy Gospel of Jesus Christ, According to St. John Bishop Richard Challoner (ed.): joh 3:21

3:21 He that doth truth: that is, he that acteth according to truth, which here signifies the Law of God. Thy law is truth. Psa. 119. 142.

Original Douay-Rheims from Project Gutenberg - Massive cleanup of OCR problems and editing done locally.