This quote comes from an earlier portion of the book–earlier in comparison to the quotes I’ve already mention, but one that is certainly relevant to what has been considered. Like the previous one, this one too is a bit long:
This revelation of the mystery is the real content of Paul’s gospel (Rom 16.26), the object of ‘the ministry which was entrusted to him’ (Col 1.25, 26; cf. Eph 3.2). Therefore Paul’s preaching itself is taken up into the great eschatological event; it is rightly and in the full sense kerygma of the gospel, that is, announcement, proclamation of the coming of salvation. That Paul’s epistles give what is no longer the first announcement of this gospel, but rather the further exposition and application of it, does not detrct from the fact that this gospel is the sole and constant subject of his epistles also; and that therefore, if one has to characterize their general content not only as kerygma, but also as doctrine and paraenesis, yet this doctrine, too, has no other object and this admonition no other starting point and ground than the fulfilling and redeeming activity of God in the advent of Christ.
-Ridderbos, Paul: An Outline of His Theology, 47-48