"the wonderful world that the Old People had lived in; as it had been before God sent Tribulation"

The Tribulation is referred to in the King James Version of the Bible in Matthew 24:21.

“For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.”

It is thought to refer to a brief phase of time where followers of God will be persecuted around the world and be purified and strengthened by it.