“Many will say to Me in that day”…They are false prophets not Christians…back up a few verses in the scripture and read it entirely. "Beware of the…“false prophets”.
The Holy Spirit does not leave because the Christian has been sealed with the King’s ring. The temple within is Holy and remains Holy for it has been sancitified forever more by the blood of the lamb, Christ Jesus.
You may have heard the idiom, “When the cat’s away, the mice will play”. If the Holy Spirit had left us, we would not have a restrainer from future sin and the guide to lead us back to repentance. Instead the Spirit is actively uplifting and testifying Christ Jesus in our hearts bringing us back to cross. Like a loving Father who watches over His children, if we disobey, there are consequences and some lead to physical death. We are still ruled in this life by the laws of sin and death, blood for blood and life for life, and ultimately we will all some day suffer the same consequence.
False prophets and false teachers are also persons who teach a different gospel than His one gospel. They are heretics. Heretics are accursed.
Galatians 1:9 Heretics actually believe that they are correct in their own interpretations of Scripture and in their own teachings. They call themselves Christians and they believe themselves to be Christians while denouncing others who do not believe as they do.
This is why it is so important to be able to discern which Church is the true Church of Christ.
Jesus promises to be with/guide His one Church (the one with Peter’s successor who alone holds the keys to the kingdom of heaven)
until the end of the age (until He returns at His Second Coming). He promised His one Church that it would be prevented from teaching error (the gates of Hades will never prevail against it).
Galatians 5:19-21
Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, 20 idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions,
heresies, 21 envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
2 Peter 2:1-3
But there were also false prophets among the people, even as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Lord who bought them, and bring on themselves swift destruction. 2 And many will follow their destructive ways, because of whom the way of truth will be blasphemed. 3 By covetousness they will exploit you with deceptive words; for a long time their judgment has not been idle, and their destruction does not slumber.