1 John 2:18-19
18 Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
The Grace New Testament Commentary - 1 John 2:18-19
2:18 Not only is "the world passing away," but what is more, the apostle and his readers are living in the last hour. Though hour can refer to a portion of a day (e.g., John 1:39; 4:6; 11:9), it also is used in reference to an undetermined length of time (e.g., John 2:4; 4:21, 23; 5:25, 28; 16:25; etc.). Here the last hour is when human history will climax with the rise (and overthrow) of Satan's final great deception.
Many interpreters take the term Antichrist as a reference to the "man of sin" who will claim godhood in the Jewish temple (2 Thess 2:3-4) and who will rule the world (Rev 13:5-8). But the many antichrists of this verse are essentially the same as the "many false prophets" of 1 John 4:1. The teachers of error are precursors of the supreme human deceiver, the Antichrist.
2:19 The "many antichrists" had once been part of the same fellowship to which the apostles themselves belonged. The word us, used four times in this verse, obviously contrasts with the "you" of the following verse, which is emphatic in Greek. Here for the first time is seen the "we" - "you" - "us" contrast (cf. 1 John 4:4-6).
The antichrists had not left the church or churches to whom John writes, for if they had they would no longer have been a problem. On the contrary, the apostle is concerned about the exposure his readers have to these men. They departed from the church, which indicated that they did not really "belong" to it in the first place.
Thomas Constable's Expository Notes on the Bible - 1 John 2:19
2:19 Those who were opposing Christ had gone out from "us." "Us" may mean the apostolic eyewitnesses, as it often does elsewhere in this epistle (cf. 1 John 1:1-5; 4:6). This would mean that these false teachers had gone out from among the apostles, not that they were apostles themselves necessarily, claiming that their message was what the apostles endorsed (cf. Acts 15:1: 2 Cor. 11:5). "Us" elsewhere in this epistle refers to the believing community (cf. 1 John 1:6-2:2), and I think it probably means that here. Some false teachers evidently had been members of local house-churches and then left them because of doctrinal differences. The physical separation of these people from the apostles and the faithful eventually illustrated their doctrinal separation from them.