Albert Barnes on John 6:39

Father's will. His purpose; desire; intention. As this is the Father's will, and Jesus came to execute his will, we have the highest security that it will be done. God's will is always right, and he has power to execute it. Jesus was always faithful, and all power was given to him in heaven and on earth, and he will therefore most certainly accomplish the will of God.

Of all which. That is, of every one who believes on him, or of all who become Christians. See Jn 6:37.

I should lose nothing. Literally, "I should not destroy." He affirms here that he will keep it to life eternal; that, though the Christian will die, and his body return to corruption, yet he will not be destroyed. The Redeemer will watch over him, though in his grave, and keep him to the resurrection of the just. This is affirmed of all who are given to him by the Father; or, as in the next verse, "Every one that believeth on him shall have everlasting life."

At the last day. At the day of judgment. The Jews supposed that the righteous would be raised up at the appearing of the Messiah. See Lightfoot. Jesus directs them to a future resurrection, and declares to them that they will be raised at the last day--the day of judgment. It is also supposed and affirmed by some Jewish writers that they did not believe that the wicked would be raised. Hence, to speak of being raised up in the last day was the some as to say that one was righteous, or it was spoken of as the peculiar privilege of the righteous. In accordance with this, Paul says, "If by any means I might attain unto the resurrection of the dead," Php 3:11.

(y) "Father's will" Mt 18:14, Jn 10:28, 17:12, 18:9, 2Ti 2:19