What does it mean that the supreme sadhu can become a supreme asādhu in a single moment, and how does this relate to the concept of not going back in existence?

According to Osho, saint and sinner are polarities within maya; whichever side you practice grows tired while the suppressed opposite grows potent, so even a supreme sadhu can flip into a supreme asādhu in an instant—and vice versa. Only the nondual state transcends both; there, no opposite remains, so there is no 'going back' in existence—oscillation ends in effortless being.