Imām Ibn Qayyim (d.751 AH) said:

It is not possible for the servant to enter paradise without purification. The servant is purified by way of four things in the worldly life:

1. repentance
2.  seeking forgiveness
3.  good deeds which wipe away evil deeds
4.  calamities which expiate the sins

If these four things are not enough to purify and cleanse the servant, he is purified by way of three things in the Barzakh:

1. The people of īmān praying over him and interceding by seeking forgiveness for him.
2.  The trials and tribulations of the grave.
3.  The charitable actions pledged on his behalf from his Muslim brothers.

If these three things are not enough to purify him, he is purified at the place of standing by way of three things:

1. The fright of resurrection and the severity of the place of standing.
2.  The intercession of the intercessors.
3.  The forgiveness of Allāh, the Mighty, and Majestic.

If these three things are not enough to purify him, then the hellfire will cleanse him and purify him from his evil. He will abide therein according to the severity of his evil and how much he possesses. When he becomes pure and good and evil has departed, he will exit the hellfire and enter paradise.

Source: Madārij al-Sālikīn 1/463
Translated by: Munīb al-Ṣumālī