When thou passest. shouldst thou pass: the habitual sense of ki with the Future. Reference to Pentateuch (Deuteronomy 31:6; Deuteronomy 31:8).
the waters... the rivers... the fire... the flame. waters... rivers... fire... flame.. general promise of future deliverance put by Figure of speech Metonymy (of Subject), for troubles of any and all kinds. This promise refers to Israel's future, and not to the Saxon race, or the Church.
kindle upon thee. pass over thee.