Evangelical scholar F.F. Bruce comments --
"And what about the 'keys of the kingdom' ? The keys of a royal or noble establishment were entrusted to the chief steward or majordomo; he carried them on his shoulder in earlier times, and there they served as a badge of the authority entrusted to him. About 700 B.C. an oracle from God announced that this authority in the royal palace in Jerusalem was to be conferred on a man called Eliakim ....(Isaiah 22:22). So in the new community which Jesus was about to build, Peter would be, so to speak, chief steward." (Bruce, The Hard Sayings of Jesus [Intervarsity Press, 1983], 143-144, as cited in Butler/Dahlgren/Hess, page 41)