The pretribulation and the advent of Our lord Jesus are two different meaning but of course related, where the first one has already been started from the time of the martyrdom of the Apostles and Christians in history up to this day. The verse that you have quoted relates to the advent or second coming of our Lord Jesus as Paul said in a “prophetic phrase” in which at that time apostasy was already taken place or happening and still happening in this day and age and hinted not to be alarmed or deceived by any man. First we need to know what is “apostasy” according to Meriam dictionary is
Definition of apostasy
: an act of refusing to continue to follow, obey, or recognize a religious faith
: abandonment of a previous loyalty : DEFECTION
History and Etymology for apostasy
Middle English apostasie, from Late Latin apostasia, from Greek, literally, revolt, from aphistasthai to revolt, from apo- + histasthai to stand — more at STAND
We are still seeing now in our time an apostasy or departure from the “true faith”. What is this “true faith”? If we go back to it’s origin, it comes from our Lord Jesus and His teachings. Who holds this faith and teachings? I will leave your discretion to answer this. This apostasy is progressing until it will reach its climax, until the anti christ or the Man of sin be revealed. The passage can be interpreted literally or figuratively as we are seeing many anti christ against the “true faith” but there will be a great one one day as these mini little anti christ are just an instruments until the anti christ gain power. Please see a further bible commentary for additional knowledge.
2 Thessalonians 2:3-4
First, &c.(2) What is meant by this falling away, (in the Greek this apostacy) is uncertain, and differently expounded. St. Jerome and others understand it of a falling off of other kingdoms, which before were subject to the Roman empire; as if St. Paul said to them: you need not fear that the day of judgment is at hand, for it will not come till other kingdoms, by a general revolt, shall have fallen off, so that the Roman empire be destroyed. The same interpreters expound the sixth and seventh verses in like manner, as if when it is said, now you know (3) what withholdeth, &c. That is, you see the Roman empire subsisteth yet, which must be first destroyed. And when it is added, only that he ho now holdeth, do hold, until he be taken out of the way; the sense, say these authors, is, let Nero and his successors hold that empire till it be destroyed, for not till then will the day of judgment come. A. Lapide makes this exposition so certain, that he calls it a tradition of the fathers, which to him seems apostolical. But we must not take the opinion of some fathers, in the exposition of obscure prophecies, where they advance conjectures (which others at the same time reject, or doubt of) to be apostolical traditions, and articles of faith, as the learned bishop of Meaux, Bossuet, takes notice on this very subject, in his preface and treatise on the Apocalypse, against Jurieux. St. Jerome indeed, and others, thought that the Roman empire was to subsist till the antichrist's coming, which by the event most interpreters conclude to be a mistake, and that it cannot be said the Roman empire continues to this time. See Lyranus on this place, St. Thomas Aquinas, Salmeron, Estius, and many others; though A. Lapide, with some few, pretend the Roman empire still subsists in the emperors of Germany. We also find that divers of the ancient fathers thought that the day of judgment was just at hand in their time. See Tertullian, St. Cyprian, St. Gregory the Great, &c. And as to this place, it cannot be said the fathers unanimously agree in their exposition. St. John Chrysostom (4), Theodoret, St. Augustine in one of his expositions, by this falling off, and apostacy, understand antichrist himself, apostatizing from the Catholic faith. And they who expound it of Nero, did not reflect that this letter of St. Paul was written under Claudius, before Nero's reign. According to a third and common exposition, by this revolt or apostacy, others understand a great falling off of great numbers from the Catholic Church and faith, in those nations where it was professed before; not but that, as St. Augustine expressly takes notice, the Church will remain always visible, and Catholic in its belief, till the end of the world. This interpretation we find in St. Cyril(5) of Jerusalem. (Catech. 15.) See also St. Anselm on this place, St. Thomas Aquinas, Salmeron, Estius, &c. In fine, that there is no apostolical tradition, as to any of the interpretations of these words, we may be fully convinced from the words of St. Augustine(6), lib. xx. de Civ. Dei. chap. 19. t. 7. p. 597. Nov. edit. where he says: For my part, I own myself altogether ignorant what the apostle means by these words; but I shall mention the suspicions of others, which I have read, or heard. Then he sets down the exposition concerning the Roman empire. He there calls that a suspicion and conjecture, which others say is an apostolical tradition. In like manner the ancient fathers are divided, as to the exposition of the words of the sixth and seventh verse, when it is said you know what hindereth; some understand that antichrist must come first. Others, that the beforementioned apostacy, or falling off from the Church, must happen before. And when St. Paul says, (ver. 7.) that he who now holdeth, do hold; some expound it, let him take care at the time of such trials, to hold, and preserve the true faith to the end. When the expositions are so different, as in this place, whosoever pretends to give a literal translation ought never to add words to the text, which determine the sense to such a particular exposition, and especially in the same print, as Mr. N. hath done on the seventh verse, where he translates, only let him that now holdeth the faith, keep it until he be taken out of the way. --- And the man of sin (7) revealed, the son of perdition, so that he sitteth in the temple of God, shewing himself as if he were God. He is called again, (ver. 8.) that wicked one....whom the Lord Jesus Christ shall kill with the spirit of his mouth. By all these words is described to us the great antichrist, about the end of the world, according to the unexceptionable authority and consent of the ancient fathers. It is as ridiculous as malicious to pretend, with divers later reformers, that the pope, and all the popes since the destruction of the Roman empire, are the great antichrist, the man of sin, &c. Grotius, Dr. Hammond, and divers learned Protestants, have confuted and ridiculed this groundless fable, of which more on the Apocalypse. It may suffice to observe here that antichrist, is to be one particular man, not so many different men. That he is to come a little while before the day of judgment. The he will make himself be adored, and pretend to be God. What pope did so? That he will pretend to be Christ, &c. (Witham) --- St. Augustine (de Civ. Dei. book xx. chap. 19.) says, that an attack would be made at one and the same time against the Roman empire and the Church. The Roman empire subsists as yet, in Germany, though much weakened and reduced. The Roman Catholic Church, notwithstanding all its losses, and the apostacy of many of its children, has always remained the same. (Calmet) The two special signs of the last day will be a general revolt, and the manifestation of antichrist, both of which are so dependent on each other, that St. Augustine makes but one of both. What presumptive folly in Calvin and other modern reformers, to oppose the universal sentiments of the fathers both of the Latin and Greek Church! What inconsistency, to give such forced interpretations, not only widely different from the expositions of sound antiquity, but also widely different from each other! The Church of God, with her head, strong in the promises of Jesus Christ, will persevere to the end, frustra circumlatrantibus hæreticis. (St. Augustine, de util cred. chap. xvii.) --- In the temple. Either that of Jerusalem, which some think he will rebuild; or in some Christian Church, which he will pervert to his own worship; as Mahomet has done with the churches of the east. (Challoner)
Nisi venerit discessio primum, Greek: e apostasia. St. Jerome (Ep. ad Algasiam. q. 11. t. 4. p. 209) Greek: Apostasia, inquit....ut omnes Gentes, quæ Rom. Imperio subjacent, recedant ab eis.
St. John Chrysostom (Greek: log. d. p. 235) says that by these words, you know what hindereth, is probably understood the Roman empire, &c. and Tertullian (lib. de Resur. Carnis. chap. xxiv. p. 340) on those words, till taken out of the way, donec de medio fiat, Quis nisi Romanorum status?
St. John Chrysostom (Greek: log. g. p. 232) Greek: ti estin e apostasia autoi kalei ton Antichriston. See Theodoret on this place.
St. Cyril of Jerusalem (Cat. xv) says, this apostacy is from the true faith and good works: Greek: aute estin e apostasia. St. Anselm and others mention both expositions, i.e. from the Roman empire, or from the faith.
St. Augustine: Ego prorsus quid dixerit, me fateor ignorare....suspiciones tamen hominum, quas vel audire, vel legere potui, non tacebo, &c. Quidam putant hoc de Imperio dictum esse Romano, &c.
Greek: O anthropos tes amartias, o uios tes apoleias, o antikeimenos, &c. ille homo peccati, ille filius perditionis: the Greek articles sufficiently denote a particular man.
Merrimack-Webster Dictionary (apostasy)