1 Jon. 2:19- They went out from us, but they did not really belong to us, because if they had belonged to us, they would have remained with us. But they went out from us to demonstrate that all of them do not belong to us.
Are you aware that this verse relates directly to Acts 15:24 - We have heard that some went out from us without our authorization and disturbed you, troubling your minds by what they said.
Do you what this was referring to?
Acts 15:1-5
1 Certain people came down from Judea to Antioch and were teaching the believers: “Unless you are circumcised, according to the custom taught by Moses, you cannot be saved.”
2 This brought Paul and Barnabas into sharp dispute and debate with them. So Paul and Barnabas were appointed, along with some other believers, to go up to Jerusalem to see the apostles and elders about this question.
3 The church sent them on their way, and as they traveled through Phoenicia and Samaria, they told how the Gentiles had been converted. This news made all the believers very glad.
4 When they came to Jerusalem, they were welcomed by the church and the apostles and elders, to whom they reported everything God had done through them.
5 Then
some of the believers who belonged to the party of the Pharisees stood up and said, “The Gentiles must be circumcised and required to keep the law of Moses.”
The "certain people" in v.1 who were teaching believers are the same as the red words in v.5.
So 1 Jn 2:19 isn't about unbelievers infiltrating the church, but rather, believers who are confused and follow false doctrines.
Just as the confused believers who claimed believers needed to be circumcised to be saved (v.1) today we have confused believers who claim that a believer must be water baptized to be saved.
Same principle, different ceremonies.