"The beholder of ineffable revelations * and recounter of the highest mysteries of God, * the son of Zebedee, * who set down in writing the Gospel of Christ, * hath taught us to theologize * concerning the Father, the Son and the Holy Spirit.
The harp of heavenly songs played by God, * the recorder of mysteries, * the divinely eloquent mouth, * doth beautifully chant the hymn of hymns; * for, moving his lips as though they were strings, * and using his tongue as a plectrum, * he prayeth that we be saved.
Proclaiming with thy thunderous tongue * the hidden word of divine wisdom, * O beloved of God, * thou ever criest out, continually moving thy lips: * In the beginning was the Word! * And thou instructest every man in the knowledge of God. ""All the Evangelists, indeed, were holy, all the Apostles, except the traitor—all were holy; yet Saint John, who wrote his Gospel last, having been sought and chosen by Christ to be in a sense his kinsman, uttered eternal mysteries with a louder trumpet. Whatever he spoke is a mystery."
--St. Ambrose of Milan, On the Sacraments (De Sacramentis) III.II.11