By revelation (κατα αποκαλυψιν). Not essentially different from δι' αποκαλυψεως (Galatians 1:12). This was Paul's qualification for preaching "the mystery" (το μυστηριον. See Ephesians 1:9).

As I wrote afore (καθως προεγραψα). First aorist active indicative of προγραφω as in Romans 15:4, not picture forth as Galatians 3:1. But when and where? Epistolary aorist for this Epistle? That is possible. A previous and lost Epistle as in 1. Corinthians 5:9? That also is abstractly possible. To the preceding discussion of the Gentiles? Possible and also probable.

In few words (εν ολιγω). Not = προ ολιγου, shortly before, but as in Acts 26:28 "in brief space or time" = συντονως (Acts 24:4), "briefly."