Didn't John say those who left were never saved?

The short answer Settled

Yes, and he was speaking of antichrists who denied the Son. They were never believers. The verse says nothing about a believer's sins or works.

Who went out

"Little children, it is the last hour; and as you have heard that the Antichrist is coming, even now many antichrists have come, by which we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us" (1 John 2:18-19).

A few verses on, John says who an antichrist is: "Who is a liar but he who denies that Jesus is the Christ? He is antichrist who denies the Father and the Son" (1 John 2:22). These were teachers who denied Christ. Later in the letter he calls them "false prophets" who "have gone out into the world" (1 John 4:1).

Their leaving showed what they were

John does not say they were of us and then stopped. He says "they were not of us," even while they were still there. Their leaving did not change what they were. It showed it: "that they might be made manifest" (1 John 2:19).

What they did not continue in was the truth about the Son, the same thing John tells his readers to keep: "let that abide in you which you heard from the beginning" (1 John 2:24).

The usual objection

Those who are truly saved "would have continued with us." So anyone who does not continue, who stops coming, stops doing good works, or falls into sin, shows he was never saved.

John is not describing believers who fell into sin. He is describing people who denied the Son, and "Whoever denies the Son does not have the Father either" (1 John 2:23). Even a believer who denies Christ out of fear is not what John describes. Peter did, and Jesus had prayed for him "that your faith should not fail" (Luke 22:32). That is on its own page.

The words behind it