I grew up as a Christian and vaguely remember from those years a verse like "if all the Jews repented at once the world would be saved". It might have been "all the Christians" or another group; it might have been "they would see God" or another concept than the "world" being "saved". I'd love pointers to where this might have come from.
The Jerusalem Talmud, also called the Palestinian Talmud, not to be confused with the Babylonian Talmud, is a collection of rabbinic writings. It includes a statement by the fourth century AD Rabbi Aha:
If Israel repents for one day, forthwith the son of David [the Messiah] will come.
According to some Jewish thought, in every generation there is someone ready to become messiah. if the circumstances of the time are right, God will appoint him, otherwise not. Another similar expression is that if Israel kept one or two Sabbaths properly, the Messiah would come.
The Christian Biblical version of this is in the second epistle of St. Peter. Christians believe that the Messiah has already come, but will return. We read in chapter 3. verse 9:
The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
This verse is in the context of the delay in the Second Coming (v 4), and explains the delay as due to patience, a desire to give more time for repentance. If God is procrastinating to give more time for repentance, what would happen if everybody did repent? There would then be no further need to delay the Second Coming. This verse, then, may suggest that if everyone repented the Second Coming would happen immediately. However, some Christians see this as merely a hypothetical inference since they believe that, in fact,it is predestined that not everybody will repent.