According to Catholicism, what particular truth will the great apostasy deny? CCC 675 Before Christ's second coming the Church must pass through a final trial that will shake the faith of many believers. The persecution that accompanies her pilgrimage on earth will unveil the "mystery of iniquity" in the form of religious deception offering men an apparent solution to their problems at the price of apostasy from the truth. The supreme religious deception is that of the Antichrist, a pseudo-messianism by which man glorifies himself in place of God and of his Messiah come in the flesh.


There are many Church teachings about the "Truth" taught by Jesus Christ in the gospel and other "Truth" handed by the Apostles as Sacred Tradition and deposited to the Church.

Is there a Catholic teachings or explanation on what particular "Truth" where apostasy will be committed?
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 4th 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".