They went out from us

They went out from us.

What does this passage mean in the epistles of John.

1 John 2:18-19 KJV
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.