In the lap of the gods - fate will decide
One very obvious suggestion for the origin of this idiom is the practice, common in many cultures since ancient times, of placing gifts on the knees of statues depicting seated gods in the hope that, in return, a prayer would be answered. Most authorities agree, however, that the phrase originated in Homer's Iliad. Patroclos, friend of Achilles, had been killed and the Trojans, having first stripped his corpse, were intending to sever the head and march with it through the city to help them gain the upper hand in the battle. It was at this point that Automedon, aware that the outcome was in the balance, said, 'These things lie on the knees of the gods.' In fact, the impending humiliation brought the sulking Achilles back into the battle and led to the rout of the Trojans and the death of Hector. The gods, it seemed, were on the side of Achilles.
See Achilles heel.