One reason Jewish people reject Jesus of Nazareth as the Messiah is because “nothing changed" after he came. Jesus didn’t immediately usher in peace for all mankind and didn’t end suffering forever. What Christians believe happens at the Second Coming, Jews believe should have happened at the First Coming if Jesus truly was the Messiah. Why are there two comings, and why was the Second Coming delayed?
The Church’s traditional understanding is that at his First Coming Christ came as Suffering Servant who redeemed mankind and made salvation possible (cf. Is 53); at his Second Coming, Christ will arrive as Conquering King who will make a new heavens and a new earth (cf. Is 11).
Why has there been, in human terms, such a long “delay" between the First and Second Comings? One possibility is that Christ is allowing us to participate in his work of salvation by our efforts to build God’s kingdom on earth in union with his perfect work. Only Christ could make atonement for the sins of all mankind and win for us universal redemption. But we can and must apply the fruits of his work to our lives and make them available to those who have yet to receive them. And, by God’s grace, the work of the Church on earth repairs the brokenness of mankind and this fallen world. In short, God does what only he can do; but he does not relieve us of the obligation to do what we can do.
This concept is similar to a Jewish religious obligation called tikkun olam, which means the “repair of the world" or the “perfection of the world." While many understand tikkun olam in a purely ethical sense of creating a just society here and now, it also has a messianic component: It is believed that performing mitzvot (following the commandments and doing good deeds) and thereby participating in tikkun olam will hasten the coming of the Messiah.