Theophylact of Ohrid

And why did Christ answer him thus: "No one is good"? Because he approached Christ as a mere man and as one of many teachers. Christ speaks as if to say: "If you consider Me good as a mere teacher, then in comparison with God no man is good; if you acknowledge Me as good as God, then why do you call Me merely a teacher?" By such words Christ wishes to convey a higher understanding of Himself, so that the man might recognize Him as God. …

Commentary on Mark ↗

Augustine of Hippo

God, therefore, is uniquely good, and this he cannot lose. He is good. He is not good by sharing in any other good, because the good by which he is good is himself. But, when a finite human being is good, his goodness derives from God, because he cannot be his own good. All who become good do so through his Spirit. Our nature has been created to attain to him through acts of its own will. If we are to become good, it is important for us to receive and hold what he gives, who is good in himself.

Letter 153, To Macedonius ↗

Hilary of Poitiers

All this is indeed as it is: He, Who is by nature God of God, must possess the nature of His origin, which God possesses, and the indistinguishable unity of a living nature cannot be divided by the birth of a living nature. Yet nevertheless the heretics, under cover of the saving confession of the Gospel faith, are stealing on to the subversion of the truth: for by forcing their own interpretations on words uttered with other meanings and intentions, they are robbing the Son of His natural unity. …

On The Trinity 9.2 ↗

Bede

But by this one God, who is good, we must not only understand the Father, but also the Son, who says, I am the good Shepherd; (John 10:11) and also the Holy Ghost, because it is said, The Father which is in heaven will give the good Spirit to them that ask him. (Luke 2:15. Vulg.) For the One and Undivided Trinity itself, Father, Son, and Holy Ghost, is the Only and One good God. …

Catena Aurea by Aquinas ↗