"Hear ye this, O house of Jacob, which are called by the name of Israel, and are come forth out of the waters of Judah, which swear by the name of the LORD, and make mention of the God of Israel, but not in truth, nor in righteousness. For they call themselves of the holy city, and stay themselves upon the God of Israel; The LORD of hosts is his name. I have declared the former things from the beginning; and they went forth out of my mouth, and I shewed them; I did them suddenly, and they came to pass. Because I knew that thou art obstinate, and thy neck is an iron sinew, and thy brow brass;" (Isaiah 48:1-4)
The waters of Judah would seem to indicate tribulation. Those who had survived and would survive the many troubles of war and captivity are encouraged to listen and to consider their spiritual condition. The Israelites were notorious for combining the worship of Jehovah with the worship of idols. In this, they swore by God's name and made mention of Him; but all was insincere. Through Jeremiah, God sharply rebuked Israel for her mixed worship.
Behold, ye trust in lying words, that cannot profit. Will ye steal, murder, and commit adultery, and swear falsely, and burn incense unto Baal, and walk after other gods whom ye know not; And come and stand before me in this house, which is called by my name, and say, We are delivered to do all these abominations? Is this house, which is called by my name, become a den of robbers in your eyes? Behold, even I have seen it, saith the LORD (Jeremiah 7:8-11).
Unlike the truly converted remnant of Isaiah 10:20, these did not lean upon God in truth but rather trusted in the false securities of lineage and position. The Jews of John's day exhibited the same heart toward God when they said within themselves, "...We have Abraham to our father (Luke 3:8)."
The former things are likely those prophecies which had been fulfilled up to this point - prophecies such as the invasion and defeat of Sennacherib. God told His people beforehand, and completed the work with rapidity; because He knew of man's tendency to discredit, in his mind, the presence of God's hand in personal and national affairs. It is possible for a man's forehead to be made strong against evil (Ezekiel 3:8), but here the context is entirely negative. The Jews followed the rebellious whims of their own hearts and refused to acknowledge God's authority. Every man is haunted by these same tendencies.