Chose (exelexato). First aorist middle (indirect), selected for himself. Israel was the chosen people.
Exalted (hupsōsen). From hupsoō, late verb from hupsos so often used of Christ.
When they sojourned (en tēi paroikiāi). In the sojourn. Late word from paroikos (sojourner, dweller, Act 7:6) common in lxx. In N.T. only here and 1Pe 1:17.
With a high arm (meta brachionos hupsēlou). Vivid picture from the lxx (Exodus 6:1, 6; Deuteronomy 5:15; Ps 136:12).