Can we fall out of heaven into hell?
The short answer
No. A person's eternal destination is settled at death. There is free will in heaven, but it will not allow a saint to do what will then be impossible for him — such as rejecting God.
Settled at death
Each person's eternal destination is determined at death.
Freedom in heaven
There is free will in heaven, but it will not let a saint do what is then impossible for him, such as rejecting God.