18 That He did not shrink from the title of good, or decline the office of master, but resented the unbelief which perceived no more in Him than body and flesh, may be proved from the difference of His language, when the apostles confessed Him their Master, You call Me Master, and Lord, and you say well, for so I am; and on another occasion, Be not called masters, for Christ is your Master. From the faithful, to whom He is master, He accepts the title with words of praise, but here He rejects the name “good master,” when He is not acknowledged to be the Lord and the Christ, and pronounces the one God alone good, but without distinguishing Himself from God, for He calls Himself Lord, and Christ, and guide to the heavenly treasures.
Source: On the Trinity (New Advent)