Or, because he sought eternal life, (and eternal life consists in such contemplation in which God is beheld not for punishment, but for everlasting joy,) and knew not with whom he spake, but thought Him only a Son of Man, therefore He says, “Why askest thou me concerning good,” calling me in respect of what you see in me, Good Master? This form of the Son of Man shall appear in the judgment, not to the righteous only, but to the wicked, and the very sight shall be to them an evil, and their punishment. But there is a sight of My form, in which I am equal to God. That one God therefore, Father, Son, and Holy Spirit, is alone good, because none see Him to mourning and sorrow, but only to salvation and true joy.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Trin., i, 13