You are getting mixed up as well as making erroneous statements.
The Day of Christ, i.e. the Day of the Lord does not take place until (1) the great falling away (from believing Jesus is the messiah) in verse 3 and (2) the man of sin revealed in verse 4.
The rapture is not the falling away. The rapture is sudden catching up, the translation of the living in Christ to meet Jesus in the air, and so ever be with the Lord.
1Thessalonians:417 Then we which are alive
and remain
shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Catholics believe that Jesus is the messiah. The great apostasy of departing that fundamental belief in Christianity has not happen yet. Will not happen until the Antichrist is anointed the King of Israel, perceived messiah by the Jews.
Paul was assuring the Thessalonians that the day of the Lord had not yet begun.
The rapture/resurrection event of 1Thessalonians4:15-18, Jesus coming for Christians, is before beginning of the day of the Lord.
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