Theophylact of Ohrid

Someone of this sort might perhaps say: if I do not believe in the coming of Christ in the flesh, but lead a life of good deeds, can I not stand together with the pious on account of these deeds? Can I not receive a reward for my works? The Apostle destroys such an objection in advance. He says: whoever rejects the coming of Christ in the flesh, let him not even think about receiving the full recompense for his works that awaits true believers, nor about being numbered among the perfectly pious. …

Commentary on 2 John ↗

Oecumenius

Whoever transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son. The Apostle first removes this objection by saying: Let no one think that by rejecting the coming of the Lord in the flesh, one can either achieve the perfect justice owed to the faithful through works or be regarded as a worshiper of God at all. …

Commentary on 2 John ↗

Bede

Everyone who goes ahead and does not remain in the teaching, etc. Note the difference in words, and embrace the truth of the faith. He says that whoever does not remain in the teaching of Christ does not have God, but whoever remains in his teaching has both the Son and the Father, to show that the Father and the Son are one true God, and to convict those of falsehood who assert that the Son is either not God, or is posterior or inferior to the Father.

Commentary on the Catholic Epistles ↗