Was kept in jail may simply be rendered as “the soldiers continued to guard Peter in jail.”

Two things emphasize the intensity of the prayer offered by the people of the church in behalf of Peter: (1) the verb itself, were praying, which indicates continuous action and (2) the adverbial expression earnestly. The expression of earnestly may be rendered as “strongly,” for example, “they were praying strongly to God” or “they were praying with words they felt in their hearts.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on The Acts of the Apostles. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .