What does it mean to "remember not the former things" in Isaiah 43:18?

(Vv. 18–21). Let Israel remember and consider God’s deliverances in the past, and thus trust God for the “new thing” about to take place in the future. In the past God made “a way in the sea” (16); in the future He will make “a way in the wilderness” for His people to return to their land. He will refresh and sustain them on their journey with rivers in the deserts, and protect them from “the beast of the field.” Thus, at last, they will “show forth” Jehovah’s praise and thus fulfil the purpose for which they were formed.

This renders the subject (namely, Israel as God's servant) as plain as His own mercy in the last days. The Jews by their idol-loving were the blind people that had eyes and the deaf that had ears. Yet were they alone of all nations Jehovah's witness and His servant. They will yet know, believe, and understand. It was because of Him Whom they rebelled against that they have been so spoiled; it is because of Him that they will be delivered, pardoned, and blessed; for as He, Jehovah, is the only God, so is He equally the sole Saviour. It was for Israel's sake that He had sent them to Babylon and brought down their pride; and new things should cause the old wonders to be forgotten when God gives water in the wilderness and rivers in the desert for His chosen people, formed for Himself to show forth His praise. It was God Who once, for their sins, "profaned the princes" and people of Israel; it is He Who for His own sake will blot out their transgressions and justify themselves. Before that day dawn, we know (what is to them still a secret) the great salvation by His grace, whereby alone He can thus righteously deal with the guilty: Christ, the cross, is the only key.

"I, I [am] Jehovah; and beside me there is no Saviour. I have declared, and I have saved, and I have shown, and there was no strange [god] among you: therefore ye [are] my witnesses, says Jehovah, that I [am] God. Yea, since the day [was], I [am] He; and there is none that can deliver out of my hand: I will work, and who shall hinder it? Thus says Jehovah, your redeemer, the Holy One of Israel, For your sake I have sent to Babylon, and I will bring down all of them as fugitives, even the Chaldeans, in the ships of their rejoicing. I [am] Jehovah, your Holy One, the Creator of Israel, your King. Thus says Jehovah, who makes a way in the sea, and a path in the mighty waters; who brings forth the chariot and horse, the army and the power — they lie down together, they shall not rise; they are extinct, they are quenched as flax — Remember ye not the former things, neither consider the things of old. Behold, I will do a new thing; now shall it spring forth: shall ye not know it? I will even make a way in the wilderness, [and] rivers in the desert. The beast of the field shall honour me, the jackals and the ostriches; because I give waters in the wilderness, [and] rivers in the desert, to give drink to my people, my chosen: the people have I formed for myself, that they might set forth my praise. Yet thou hast not called upon me, O Jacob; for thou hast been weary of me, O Israel. Thou hast not brought me the small cattle of thy burnt-offerings; neither hast thou honoured me with thy sacrifices. I have not made thee to toil with oblation, nor wearied thee with frankincense. Thou hast bought me no sweet cane with money, neither hast thou filled me with the fat of thy sacrifices; but thou hast made me to toil with thy sins, thou hast wearied me with thine iniquities. I, I [am] he that blots out thy transgressions for mine own sake; and I will not remember thy sins. Put me in remembrance; let us plead together: set thou forth [thy cause], that thou mayest be justified. Thy first father sinned, and thine interpreters have transgressed against me. Therefore I profaned (or, will profane) the princes of the sanctuary, and made (or, will make) Jacob a curse, and Israel a reviling" (vv. 11-28).