Beloved, follow not that which is evil, but that which is good - There can be no doubt that in this exhortation the writer had Diotrephes particularly in his eye, and that he means to exhort Gaius not to imitate his example. He was a man of influence in the church, and though Gaius had shown that he was disposed to act in an independent manner, yet it was not improper to exhort him not to be influenced by the example of any one who did wrong. John wished to excite him to acts of liberal and generous hospitality.
He that doeth good is of God - He shows that he resembles God, for God continually does good. See the sentiment explained in the notes at 1-John 3:7.
He that doeth evil hath not seen God - See the notes at 1-John 3:8-10.

Follow not that which is evil - Μμ μιμου το κακον· Do not imitate that wicked man, i.e., the conduct of Diotrephes; be merciful, loving, and kind. For whatever profession any man may make, it will ever appear that he who doeth good is of God - he alone is the person who uses rightly the grace received from God, and he alone shall enjoy the Divine approbation;
While he that doeth evil - He who is unfeeling, unmerciful, unkind, hath not seen God - has no proper knowledge of that God whose Name is mercy, and whose Nature is love.

Beloved, follow not that which is evil, but that which is good. He that doeth good is of God: but he that doeth evil hath not (e) seen God.
(e) Has not known God.

Beloved, follow not that which is evil,.... Follow not evil in general, it being hateful to God, contrary to his nature and will, and bad in itself, as well as pernicious in its consequences; and particularly follow not, or do not imitate the particular evil or evils in Diotrephes; as his pride, ambition, love of preeminence, and tyrannical government in the church, and especially his hard heartedness, cruelty, and inhospitality to the poor saints; and so the Arabic version reads, "do not imitate him in evil"; the examples of persons in office and authority have great influence, especially in cases of charity, when men can be excused thereby, and save their money, or be freed from an expense:
but that which is good; follow and imitate that, be a follower of God, imitate him in acts of kindness and beneficence, be merciful as he is; copy the deeds of Jesus Christ, who went about doing good, and declared it to be more blessed to give than to receive; and tread in the steps of those good men, who have shown love to the name of Christ, by ministering to his saints; for though the apostle may mean everything that is good, which is to be followed and imitated in any, yet he chiefly designs acts of kindness and beneficence to poor saints and ministers: to which he encourages by the following,
he that doeth good is of God; he is a child of God, he appears to be so, in that he is like to his heavenly Father, who is kind and merciful; he is born of God, he is passed from death to life, which his love to the brethren shows; he has the grace of God, and strength from Christ, and the assistance of the Spirit, without either of which he could not do that which is good:
but he that doeth evil hath not seen God; has had no spiritual saving sight of God in Christ; for if he had, he would abhor that which is evil, and, with Job, abhor himself for it, and reckon himself, with Isaiah, as undone, Job 42:6, for such effects has the sight of God on the souls of men; such an one knows not God, nor what it is to have communion with him: for those who live in sin, in whom it is a governing principle, cannot have fellowship with God; nor has such an one ever felt the love of God in his soul, or been made a partaker of his grace, which would teach and constrain him to act otherwise. Compare this text with 1-John 3:10, which shows the Apostle John to be the writer of this epistle. The Ethiopic version reads, "shall not see God"; that is, hereafter, in the world to come.

follow not that which is evil--as manifested in Diotrephes (3-John 1:9-10).
but . . . good--as manifested in Demetrius (3-John 1:12).
is of God--is born of God, who is good.
hath not seen God--spiritually, not literally.

Follow not that which is evil - In Diotrephes. But that which is good - In Demetrius. He hath not seen God - Is a stranger to him.