a) Because in every man there is sin, and sin is a wound that does not heal by itself, without medicines; and in the case of some people this wound is so deep and dangerous that it can be healed only by cauterisation and amputation. That is why no one can be cleansed of his sins without spiritual sufferings.
b) Sin is the most horrible impurity and abomination in the eyes of God; but nothing abominable, vile, and unclean can enter the Kingdom of Heaven. Wherever you put a person suffering from an internal disease or oppressed with cruel sorrow, he will suffer, even if he is put in the most magnificent palace; that is because his disease and sorrow are always and everywhere with him and in him.
It is the same in the case of a sinner who is impenitent and not cleansed of sins — wherever you put him, he will suffer even in Paradise itself, because the cause of his suffering (i.e., sin) is in his heart. To a sinner everywhere will be hell. On the other hand, whoever feels real, heartfelt joy will rejoice both in a palace and in a hut, and even in prison, because his joy is in his heart.
So too for a righteous man whose heart is filled with consolations of the Holy Spirit, wherever he may be, everywhere will be Paradise because the Kingdom of Heaven is within us (Luke 17:21). However much you cut off the branches of a living tree, it will not die, but will against produce new branches and in order to destroy it completely you must tear it out of the ground by its roots.
In exactly the same way, you cannot destroy sin from the human heart by lopping off or giving up a few vices or habits; and therefore whoever wishes to destroy sin from the heart must tear out the actual root of sin. But the root of sin is deeply embedded in the human heart and firmly attached to it, and therefore it is quite impossible to eradicate it without pain. And unless the Lord had sent us the Great Physician, Jesus Christ, no one could have destroyed the root of sin, and all efforts and attempts to do so would have proved absolutely futile.”
This selection is from Indication of the Way into the Kingdom of Heaven by St. Innocent of Alaska (Jordanville, NY: Holy Trinity Monastery, 2006), 29-30. This text was originally written in the Aleut language.