So the sheep of God or Jesus should immediately recognize false teachers / doctrine
Yes, they should. However, that's not always the case. I came out of catholicism and joined a pentecostal church (Volle Evangelie , full gospel) when a was about 20. There was a lot of truth and I was told you should be baptized in the holy Spirit and as a result I would (or could) speak in tongues. I was told some truth, half-truths and lies. It is a long story.
I thought I could speak in tongues, however I didn't do that a lot.
The sheep of God also don't need a warning.
Yes, they do sometimes! I needed it. I learned a lot from Christian sites on the internet. Non-charismatic sites. It took me some time to study the whole subject about speaking in tongues. Finally I had to leave the charismatic (Rafael) denomination I was part of for about 20 years.
I wished I was warned long time ago. I warning have warned other members from that Rafael-church, it is very hard to reach them.
Most churches teach some error. Like Calvinism, Pretrib rapture (Opname voor de Grote Verdrukking), infant baptism, new-age influences.
And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plaques. Rev 18:4Are you part of a local church?