18. grow—Not only do not "fall from" (2Pe 3:17), but grow onward: the true secret of not going backward. Eph 4:15, "Grow up into Him, the Head, Christ."
grace and … knowledge of … Christ—"the grace and knowledge of Christ" [Alford rightly]: the grace of which Christ is the author, and the knowledge of which Christ is the object.
for ever—Greek, "to the day of eternity": the day that has no end: "the day of the Lord," beginning with the Lord's coming.