The Greek here uses two verbs, μέλλομεν (ESV: we were) and θλίβεσθαι (ESV: to suffer affliction). The first one can function more or less like an auxiliary verb in English (we will
or we are about to
) with the primary meaning conveyed by the second verb (to suffer
). However, μέλλομεν combined with a verb in the present tense of the infinitive (as is the case here) may refer to an action that necessarily follows a divine decree.
This fits the context well, and we could translate the first verb as we are destined.
1 The second verb does not usually denote persecution explicitly, but refers more generally to affliction and hardship (which may, of course, result from persecution).2
4 For when we were with you, we kept telling you beforehand that we were to suffer affliction, just as it has come to pass, and just as you know.