If while living among the nations, to whom Jehovah their God would drive them, the children of Israel and their children would return to Him and listen to His voice with all their heart and with all their soul, He would turn their captivity and be compassionate to them, and He would gather them from all the peoples among whom He had scattered them and bring them into the land which their fathers possessed and would do good for them and multiply them (30:1-5). He would circumcise their heart (v. 6a), meaning that He would somewhat transform their rebellious nature. They would then love Him with all their heart and with all their soul (v. 6b). He would put all the curses upon their enemies and would give them an excess of prosperity in all their undertakings, and would exult over them for good (vv. 7-10).
This promise, which is a prophecy, has not yet been fulfilled, but we are waiting for the fulfillment. The world situation is very advanced with respect to the fulfillment of this prophecy. Eventually, the Jews will rebuild the temple, and even now they are making preparations for this. Soon after they have recovered the site of the old temple, the temple will be erected. When we see these things taking place, we will know that the "summer" of the time of restoration is near (Matt. 24:32 and notes). We should all be prepared for this and should watch and pray.