As Stephen recounts the history of Israel to his persecutors, he refers in passing to several chronological details. Abraham’s descendants were slaves for 400 years (Acts 7:6), God appeared to Moses after 40 years of sojourn in Midian (7:30), and another 40 years passed with Israel in the wilderness (7:36). Stephen thus highlights the parallel, already evident in Exodus, between the experience of Moses and that of Israel, the 40 years of the “head” recapitulated in the 40 years of the “body.”
Paul repeats some of the same chronological details in his first sermon in Acts 13. Yahweh bore the grumbling of Israel for 40 years (13:18) and there were 450 years between the promise to Abraham and the entry into the land (13:19). Paul adds that Saul reigned for 40 years (13:21).