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The reason that the church has become unsettled is a misunderstanding over the return of the Lord and “our gathering” to him. The word for the coming of the Lord is παρουσία, the most common word for the return of Christ.
The noun παρουσία simply means “presence” or “arrival,” and is used in a variety of ways. The noun can refer to the arrival of a human (Paul in 2 Cor 7:6), but it is also used for the visit of a person of high ranking, such as a king (3 Macc 3:17). This use usually included flattery, tributes, delicacies, transportation, and gifts of golden wreaths or money. If a god was active in history helping a human that presence of the god is called a παρουσία. Josephus uses the word to describe God’s presence in helping Israel (Antiq. 3.80). The word is used often in connection with sacred events where the presence of a god is assumed.
Paul uses this word not only to refer to the presence of Jesus, but also of the Man of Lawlessness (the Anti-Christ and has his own anti-parousia). The word can be stretched to cover all of the events associated with the eschatological age, similar to the “day of the Lord” in the Hebrew Bible.
The second word, “gathering” is ἐπισυναγωγή, is quite rare in the New Testament, used only here and in Hebrews 10:25 where it refers to the gathering together of believers for worship. The obvious meaning of the term here is that Paul is referring to the Rapture, using similar terminology to 1 Thess 4:13-18.
There are a number of Old Testament passages that teach that Israel will be re-gathered prior to the Messianic kingdom. For example, in Isa 43:4-7 God gathers the children of Zion from the east, west, north, and south, a clear reference to Jews living in the Diaspora. When the eschatological age begins, God will gather his elect (the chosen) from the four winds and bring them back to Zion. (Compare this to LXX Isa 52:12, God is the “gatherer of Israel.” See also Isa 56:8; Jer 31:8. Ezek 20:34; 34:16, Ps 106:47.)
This noun is used in the Second Temple Period for the gathering of Israel at the beginning of the eschatological age. In 2 Macc 2:7 the secret place where the Ark is hidden will not be revealed until “God gathers his people again” (using the verb συνάγω and the noun ἐπισυναγωγή). The word also appears in T.Naph 8:3 where it describes the gathering of the righteous out of the nations at the beginning of the eschatological age. Similarly, T.Ash 7:7 the Lord will gather Israel on account of Abraham, Isaac and Jacob.
The idea of Israel being re-gathered is the point of Jesus’ words in Matt 23:37 / Luke 13:34. Jesus contrasts God’s desire to gather Israel together under his wings with their rejection of him as the Messiah. A bit later in the Gospels Jesus uses the noun to describe the gather of the elect from the four winds when Messiah judges the world (Matt 24:31). In fact, in Matthew there is a loud trumpet call that draws the elect from the four corners of the world. The parallel is not precise, however, since Jesus is referring to the gathering of Jews in dispersion together just prior to the establishment of the kingdom. Paul is addressing a Gentile congregation
It is better, therefore, to see Paul’s use of the word as an extension of the Jewish idea of a gathering together prior to the coming of Messiah. Prior to the Day of the Lord there will be a “gathering” which is described more fulling in 1 Thess 4:13ff. Paul takes other elements of Jewish theology and tradition and expands them in the new Christian ways in this present age of Grace, this appears to be another of these adaptations.