The Holy Spirit Withdrawn
God said from the beginning that the Holy Spirit won't strive with us forever.
And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
- Genesis 6:3 (KJV)
The danger is very real here. If God withdraws His Spirit, then another spirit enters.
I recently ran into this video that goes into great detail about this.
It's a very sobering video, and I strongly suggest that you watch it.