And I told you which one it couldn’t be.
Jesus said, “Those who are not against us are for us.”
That’s early in the gospel of John. Should I look it up. Therefore, it’s inappropriate to put restrictions on saying that all true believers are His church.
I would encourage you to look at the notion advanced in Rev 2 that a “church” can have its candlestick removed. In other words, it is no longer “light,” so why waste a perfectly good candlestick.
If only the early church had known this, they could have avoided all those conflicts with the Arians, Nestorians, Eutychians, etc. Determining who is in or out of the Church would have been as simple as saying: "Are you for Jesus or against him?". All the heretics would have answered "Yes", and then the Catholics and the heretics would all have been one big happy family.
-A8