Originally posted by @eladar
They probably get it from 1 John
By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God;
Intellectual assent does not a Christian make.
The verse you quoted assumes the believer has already trusted Jesus as saviour and is born again. Having been born again means the believer has the spirit of God and is subsequently able to know the difference between "the spirit of truth, and the spirit of error". Verse 6 chapter 4.
After all, the book was written to believers, not unbelievers. Without the spirit of God it is impossible to know God.