Someday, after mastering the winds, the waves, the tides and gravity, we shall harness for God the energies of love, and then, for a second time in the history of the world, man will have discovered fire.

This quote means that one day, people will be able to use the power of love to do great things. It is saying that just as humans once discovered fire and used it to make progress, understanding the power of love can lead to more progress and change. Love will become a source of energy, where humans can draw from to help make the world a better place.