worship song by Hillsong. The phrase "mighty to save", is taken from Zephaniah 3:17 which says, "The LORD your God is with you, He is mighty to save. He will take great delight in you, He will quiet you with His love, He will rejoice over you with singing."
In the context above, Judah is condemned to be destroyed because of their unfaithfulness. However, the obedience of King Josiah postponed this judgment until shortly after his reign. The passage is thus a prophetic one, alluding to Christ who would ultimately come to save His people from this judgment. Jesus' sacrifice for us on the cross brings us salvation if only we would believe. Jesus is indeed mighty to save.