Isaiah 55:3

Again the Lord urged the hearers (everyone) to come to Him. He pressed them to listen to what He was saying, twice. God Himself is the feast. The result for them would be life, real life as opposed to the vain life described above (Isaiah 55:2). Real life would involve living under an everlasting covenant that God would make with His people. This is probably a reference to the New Covenant, since the implication is that God would make it in the future (cf. Isaiah 54:10).

While Jeremiah 31:31 says that Yahweh would make a new covenant "with the house of Israel and with the house of Judah," that covenant is the one under which all the people of God have lived since Jesus ratified it (2 Corinthians 3:6; Hebrews 8:8-12). Its benefits are not all exclusively for Israel, though some of its benefits are exclusively for Israel and these benefits will only come into Israel’s possession in the Millennium. Jesus terminated the Mosaic Covenant (Mark 7:19; Romans 10:4; Romans 14:14; Hebrews 8:6 to Hebrews 9:22; et al.) and ratified the New Covenant (Luke 22:20; 1 Corinthians 11:25) with His blood when He died on the Cross.

However, this could be a reference to the Davidic Covenant, which is also eternal (cf. 2 Samuel 7:16). [Note: J. Martin, p. 1110; Dyer, in The Old . . ., p. 576.] This new covenant would be in full harmony with God’s promises to David, in the Davidic Covenant, regarding David’s descendant who would rule over his house forever (2 Samuel 7:12-16; 1 Chronicles 17:23-26; Psalms 89:35-38; cf. Isaiah 9:6; Luke 1:32-33; Acts 13:34).