Then it must not be part of the church for xzins, since it didn't come BEFORE St Paul told the Thesselonians not to forget what he'd taught them.
St. Paul was martyred sometime between 65 and 67 AD. The EARLIEST accepted date for any of the Gospels is Mark in about 65 AD. NONE of the other Gospels or anything other than the Pauline epistles were written prior to Paul's death. This is not "Catholic tradition," this is fact supported by other early Church writings and essentially ALL Biblical scholars. To illustrate, Acts describes Paul's martyrdom, it COULDN'T have been written during his lifetime.