Exegesis:

apobēsetai humin eis marturion ‘it will result for you in a testimony.’ For apobainō cf. on 5.2.

marturion (cf. on 5.14) here either in a passive sense, ‘testimony for you,’ i.e. it will bring you a good reputation, or in an active sense, ‘testimony,’ ‘testifying,’ i.e. it will bring you an opportunity to testify. The latter is preferable.

Translation:

This will be a time for you, preferably, ‘this will lead you,’ ‘its-consequence (will be) that you’ (cf. Balinese), ‘these things will carry you’ (Sranan Tongo), ‘this will give you the opportunity’ (cf. New English Bible).

To bear testimony, i.e. to tell publicly about the things you have experienced while being with me; hence, ‘give testimony in my favour’ (cf. Bahasa Indonesia), ‘speak on my behalf before them’ (Kele); or, shifting to the contents of the testimony, “declare your faith” (The Four Gospels – a New Translation), “tell the Good News” (Good News Translation). For testimony.

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.