Matthew 24:13 tells us that those who endure to the end will be saved. A lack of total commitment has terrible consequences. As Luke 9:62 says, no one who looks back is fit for the Kingdom of God. If people fall away after receiving God's Spirit, they will not repent.
The word "impossible" is used there. That is a very powerful, strong word. What a terrifying thought to consider. If they know Christ but return to the world, their end is worse than the beginning, as the apostle Peter informs us in II Peter 2:20.