Jamieson, Fausset and Brown on 2 John 1:9

9. The loss (2Jo 8) meant is here explained: the not having God, which results from abiding not in the doctrine of Christ.

transgresseth--The oldest manuscripts and versions read, "Every one who takes the lead"; literally, "goes," or "leads on before"; compare Joh 10:4, "He goeth before them" (not the same Greek). Compare 3Jo 9, "Loveth to have the pre-eminence."

hath not God--(1Jo 2:23; 5:15). The second "of Christ" is omitted in the oldest manuscripts, but is understood in the sense.

He--emphatical: He and He alone. 10. If there come any--as a teacher or brother. The Greek is indicative, not subjunctive; implying that such persons do actually come, and are sure to come; when any comes, as there will. True love is combined with hearty renunciation and separation from all that is false, whether persons or doctrines.

receive him not ... neither bid him God speed--This is not said of those who were always aliens from the Church, but of those who wish to be esteemed brethren, and subvert the true doctrine [Grotius]. The greeting salutation forbidden in the case of such a one is that usual among Christian brethren in those days, not a mere formality, but a token of Christian brotherhood.