According to Catholicism, what particular truth will the great apostasy deny?

According to Catholicism, what particular truth will the great apostasy deny?

The great apostasy are link with the false prophet and the prophecy of the Antichrist. Also, the false prophets in the last hour according to St. John belong to the Church but separated themselves indicating schism.

Little children, it is the last hour; and as you have heard that Antichrist cometh, even now there are become many Antichrists: whereby we know that it is the last hour. They went out from us, but they were not of us. For if they had been of us, they would no doubt have remained with us; but that they may be manifest, that they are not all of us. (1 John 2:18-19)

Since we are dealing with the old members of the Church in the form of Pastors like the Apostles, in our time it will be the equivalent of Bishops or Cardinals. This Pastors are infected by the spirit of Antichrist.

What would the Antichrist deny? St.John told us that it will deny the Father and the Son of God?

Who is a liar, but he who denieth that Jesus is the Christ? This is Antichrist, who denieth the Father, and the Son. (1 John 2:22)

The inspired reflection of Hans Urs Von Balthasar in his famous book "Dare we hope that all men be saved" teaches us the reflection on the Will of the Father to save all mankind (1 Timothy 2:4) and that the Father sent Jesus Christ not to condemn the world but to save all souls. (John 3:17)

So the Gospel as good news teaches the Truth that God the Father desire the salvation of all man. St. John Paul II warned us of the coming confrontation with the false prophet preaching the "anti-gospel".

What is the gospel of Christ? Pope Benedict XVI said:

"The heart of the gospel is Divine Mercy"

Pope Francis Bull titled Miserecordiae Vultus teaches that:

The Church is commissioned to announce the mercy of God, the beating heart of the Gospel, which in its own way must penetrate the heart and mind of every person. The Spouse of Christ must pattern her behaviour after the Son of God who went out to everyone without exception. In the present day, as the Church is charged with the task of the new evangelization, the theme of mercy needs to be proposed again and again with new enthusiasm and renewed pastoral action. It is absolutely essential for the Church and for the credibility of her message that she herself live and testify to mercy. Her language and her gestures must transmit mercy, so as to touch the hearts of all people and inspire them once more to find the road that leads to the Father.

The Church’s first truth is the love of Christ. The Church makes herself a servant of this love and mediates it to all people: a love that forgives and expresses itself in the gift of oneself. Consequently, wherever the Church is present, the mercy of the Father must be evident. In our parishes, communities, associations and movements, in a word, wherever there are Christians, everyone should find an oasis of mercy.

Vatican II was inspired to preach the Mercy of God. The New Evangelization Mission of the Church in the Third Millennium is to preach the gospel of Mercy in the four corners of the world, so that all people will come to knowledge of Truth of who Jesus is, the Redeemer of all mankind.

Will the Antichrist and the false prophets or the Bishops Cardinals infected by the spirit of Antichrist be inspired by Satan to oppose this mission?

St. Faustina diary will enlighten us on the spirit of Antichrist:

I have now learned that Satan hates mercy more than anything else. It is his greatest torment. ... The glory of the Divine Mercy is resounding, even now, in spite of the efforts of its enemies, and of Satan himself, who has a great hatred for God's mercy. This work will snatch a great number of souls from him, and that is why the spirit of darkness sometimes tempts good people violently, so that they may hinder the work. (Diary, 764 and 1659)

So, St. Faustina revelation is the key connection on the words of St. John Paul II saying:

“We are now standing in the face of the greatest historical confrontation humanity has gone through. I do not think that wide circles of American society or wide circles of the Christian community realize this fully. We are now facing the final confrontation between the Church and the anti-Church, of the Gospel versus the anti-Gospel. - The Final Confrontation

How can we connect this to St. John words in 2 John 1:7?

For many seducers are gone out into the world, who confess not that Jesus Christ is come in the flesh: this is a seducer and an antichrist.

Jesus is Divine Mercy made flesh, and if we deny the Mercy of God on a soul seeking healing and forgiveness we deny the Son of God in the flesh.

Pope Francis teachings on Amoris Laetetia and CCC2267 magnifies the Mercy of God on all souls like the couple in irregular union, the convicted criminals sentenced to death from heinous crimes, and the Pan Amazon Synod the indigenuos people are being reach out by the Church to preach the Mercy of God in these end times. Even the LGBTQ are "children of God" according to St. Mother Theresa. No sin is greater than the Mercy of God.

The Mercy of God is an abyss and no one is beyond redemption.

It is noteworthy to note that CCC2267 stated the changes in doctrines are "in the light of the Gospel" putting the connection on the preaching of the Gospel vs. anti-gospel.

Consequently, the Church teaches, "in the light of the Gospel", that “the death penalty is inadmissible because it is an attack on the inviolability and dignity of the person”, and she works with determination for its abolition worldwide.

According to Catholicism, what particular truth will the great apostasy deny?

The Great Apostasy is the denial of the Mercy of God by the willfull rejection, contradiction and opposition of the approved Magisterial Teachings of Pope Francis centered on the Mercy of God.

The Church was commissioned by Christ to preach the gospel in the whole world and the Church mission is salvation of all souls.

The scriptures teaches that the Church must upheld "the primacy of mercy over justice." (James 2:13)

For judgment is without mercy to one who has shown no mercy. Mercy triumphs over judgment.

Cardinal Manning, the most prominent father at the First Vatican Council who was responsible for the definition on papal infallibility, lists some signs of "the persecution which Antichrist shall inflict upon the Church of God," in the 4^th lecture of his Temporal Power of the Vicar of Christ, "The Perpetual Conflict of the Vicar of Christ" (all 4 lectures in printed form from TradiBooks as The Pope and The Antichrist), pp. 147ff.:

"1. The first sign or mark of this coming persecution is an indifference to truth." (p. 148)

and

"2. The next step which indicates the fulfilment of the prophecies is the persecution of the truth." (p. 151)

These would seem to be an "apostasy from the truth".

^{(adapted from this answer to "How can one identify the Anti-Christ, can he be a Bishop opposing the Papacy in relation to 1 John 2:19-20?")}

The philosophical foundation of the heresy of Modernism is agnosticism, that one cannot know (ἄ = "not", γνωστος = "to know"). Commenting on Pius XII's anti-Modernist encyclical Humani Generis, Fr. Réginald Garrigou-Lagrange, O.P. writes that Modernists have redefined what truth is:

Finally, [Modernists] consider the traditional definition of truth «chimerical»: «Adaequatio rei et intellectus», the conformity of judgement with extra-mental being and with its immutable laws, and they want to «substitute for it» this new definition: Conformitas mentis et vitae, the conformity of our judgment with life and with its subjective exigencies, and this leads to an «insufficiently objective certainty» regarding the existence of God, as in the proof proposed by Kant.

To redefine truth could be considered an "apostasy from the truth".