Come unto me (comp. Isaiah 55:1, "Come ye to the waters"). God dispenses the waters (see Isaiah 44:3). I will make an everlasting covenant with you. That the "everlasting covenant" once made between God and man had been broken by man, and by Israel especially, is a part of the teaching contained in the earlier portion of Isaiah (Isaiah 24:5).
We find the same asserted in the prophecies of his contemporary, Hosea (Hosea 6:7). It would naturally follow from this that, unless God gave up man altogether, he would enter into a new covenant with him.
Accordingly, this new covenant is announced, both in Hosea (Hosea 2:18-20) and in the later chapters of Isaiah, repeatedly (Isaiah 42:6; Isaiah 49:8; Isaiah 54:10; Isaiah 4:3; Isaiah 56:4, Isaiah 56:6; Isaiah 54:1-17:21; Isaiah 61:8).
Having been thus set before the nation, it is further enlarged upon by Jeremiah (Jeremiah 31:31-33; Jeremiah 32:40; Jeremiah 11:5) and Ezekiel (Ezekiel 16:60-62; Ezekiel 34:25; Ezekiel 37:26-28). Almost all commentators allow that the Christian covenant is intended—that "new covenant" (Hebrews 9:15) under which man obtains pardon and salvation through the Mediatorship of Christ.
Even the sure mercies of David. The "sure mercies of David" are the loving and merciful promises which God made to him. These included the promise that the Messiah should come of his seed, and sit on his throne, and establish an everlasting kingdom (Psalms 89:2-5, Psalms 89:19-37), and triumph over death and hell (Psalms 16:9, Psalms 16:10), and give peace and happiness to Israel (Psalms 132:15-18).
The promises made to David, rightly understood, involve all the essential points of the Christian covenant.