“Not only that,
We also boast
In our sufferings,
Knowing that suffering
οὐ μόνον δέ, ἀλλὰ καὶ καυχώμεθα ἐν ταῖς θλίψεσιν, εἰδότες ὅτι ἡ θλῖψις ὑπομονὴν κατεργάζεται,
Paul said that not only that (οὐ μόνον δέ), we also boast (ἀλλὰ καὶ καυχώμεθα) in our sufferings (ἐν ταῖς θλίψεσιν), knowing that (εἰδότες ὅτι) suffering (ὅτι ἡ θλῖψις) produces (κατεργάζεται) endurance (ὑπομονὴν). Only the Pauline letters used this word καυχώμεθα, that means to boast, glory, or exult proudly. Paul said that they would be happy or boast in their sufferings because these sufferings would produce endurance or lead to preservation in their beliefs. This is a little like a Stoic concept. Do you believe that suffering produces endurance?