Exegesis:

legō humin ‘I tell you,’ cf. on 3.8.

en tachei lit. ‘with speed,’ hence ‘quickly,’ ‘speedily.’

plēn ‘but,’ here marking stronger contrast than alla, or de.

ho huios tou anthrōpou elthōn ‘the Son of man, when he comes,’ implying that the coming of the Son of man is God’s way of providing justice for his elect. The place of the subject with the participle that goes with it, before the interrogative participle ara which modifies the subsequent clause, is awkward and suggests strong emphasis.

ara heurēsei tēn pistin epi tēs gēs ‘will he find faith on earth?’ ara (†, to be distinguished from ara, cf. e.g. 1.66) suggests a note of anxiety. The article tēn before pistin is either generic and then better left untranslated (cf. Revised Standard Version), or points to a specific form of faith, i.e. the faith that is expecting the Son of man. This is preferable.

Translation:

When … earth. Transposition of the two clauses may be preferable, e.g. “will the Son of Man find faith on earth when he comes” (Good News Translation, similarly Bahasa Indonesia, Batak Toba). — Find faith, or, “find men … who believe in him” (Phillips, similarly Tae’ 1933).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.