First of all, we need to remember that St. Faustina's Diary, beautiful and profound as it is, is not the equivalent of the inspired and inerrant Holy Scriptures. When our holy Mother the Church gives its nihil obstat to the writings of a saint, therefore, she does not thereby proclaim that every word the saint wrote is the pure, infallible Word of God — only that there is nothing in the saint's writing in question that clearly contradicts the definitive teachings of the Church on faith and morals.
The reflections and opinions of a saint — even their visions and meditations — can sometimes be colored by their own cultural background and blind spots, and their own psychological make-up (on this, see Fr. Benedict Groeschel's book, A Still Small Voice).
In general, of course, we should give the benefit of the doubt to a saint, because he or she is filled to overflowing with the Holy Spirit, the Spirit of Truth. But sometimes a saint can form opinions or make speculations that go beyond what the Church has been willing to define about one or more of the mysteries of the Faith.
To some extent, that is the case here. Saint Faustina claims she had a vision of hell — and no doubt she did. But whether she understood that vision entirely clearly, or whether it was colored, to some extent, by her own expectations or speculations or cultural assumptions, is hard to say.
The Church has not defined that the souls in hell suffer torments of the flesh directly corresponding to the manner in which the soul sinned in the flesh, nor has the Church defined that there is a spiritual "fire" that penetrates the soul that is "lit by God's anger." Nor for that matter has the Church defined that the torments of hell include a literal, physical fire (which, in fact, St. Faustina does not mention). These things are matters for theological speculation, and the content of St. Faustina's vision is not decisive evidence on these matters for the Church.
Secondly, we need to interpret St. Faustina's frightening vision of hell in the context of the rest of her Diary. In entry 1588, for example, our Lord told her that He is reluctant to mete out retributive justice to sinners. "I do not want to punish aching mankind but to heal it, pressing it to My merciful Heart," He says. "I use punishment when they force Me to do so; My hand is reluctant to take hold of the sword of justice."
First, the author discredited Sister Faustina's vision of Hell by saying that "[t]he reflections and opinions of a saint — even their visions and meditations — can sometimes be colored by their own cultural background and blind spots, and their own psychological make-up." [3] His support for his comment was from a book written by a priest. He apparently considered his source to be irrefutable truth, unaffected by the source's own blind spots and perhaps even envy of those blessed to see Heavenly visions.
Secondly, the author, after discrediting Sister Faustina, relied on this statement, also by Sister Faustina, a statement that apparently was not deemed by the author as part of the saint's "reflections and opinions" "colored by...cultural background and blind spots..." [3] when such a statement seemed to bolster his position: "'I do not want to punish aching mankind but to heal it, pressing it to My merciful Heart,' He says. 'I use punishment when they force Me to do so; My hand is reluctant to take hold of the sword of justice.'" [4] [Emphasis original].
The author contradicted himself by finding Sister Faustina both to be disbelieved on the one hand and to be believed on the other. This is an example of Satan's failures. Satan can never be consistent because it lies. Only God's Truth is unchanging and eternal. Satan's lies keep changing to fit within a circumstance, to give it a specious appearance, in order to mislead souls and send them to eternal damnation.
Is Sister Faustina's vision of Hell in conflict with what God said? No. What God said reinforces Sister's Faustina's vision of Hell. The tortured souls in Hell had forced God to punish them even though God is very much reluctant to do so. At the point where God had to "take hold of the sword of justice" [5] is where Divine Mercy is maximized and where it is subjected to the reluctant force of Divine Justice.
Thirdly, the author also included this: "'I received a deeper understanding of divine mercy,' she writes. 'Only the soul that wants it will be damned, for God condemns no one' (Diary, 1452). This fits very well with the Catechism's definition of hell as "a state of definitive self-exclusion from communion with God" (Catechism, 1033)." [6]
The above does not mean that God will not exercise Divine Justice, for it has always been man's Free Will to reject God and those souls that reject God do not care whether or not they are dammed--it is not that God wanted them dammed. God wants them saved but those souls in Hell had chosen otherwise.
In the end, the author found himself nowhere to hide from God's Truth and relented. This is one of his concluding thoughts: "The doctrine of Hell, therefore, though it is disconcerting and sad to us, does not contradict the merciful love of God." [7]
Additional thought added February 6, 2019: This Satan surrogate in the sentence quoted above spoke of Hell being a "doctrine" when Hell is not a "doctrine" but a very real place. Again, Satan cannot hide its appearance for long; it cannot, because God will not permit it. Even if Hell were a "doctrine," how can it be "disconcerting" since it is part of God's creation? And who is "us" that the author indicated were "sad"? Could "us" refer the author himself and Satan? [8] Were they upset and unhappy because Hell had be exposed and they could no longer cover it up with omissions and deceits?
[1] https://www.thedivinemercy.org/library/article.php?NID=2690
[2] Ibid.
[3] Ibid.
[4] Ibid.
[5] Ibid.
[6] Ibid.
[7] Ibid.
[8] Ibid, everything in quotation marks refers back to the article cited under note [1].
