What is the work which God the Father gave to Jesus?
This was the salvation of all those whom God had chosen.
Why did the Mediator need to be divine?
Two reasons are given. First, because a creature would not have survived the wrath of God. Only a divine Person would be able to endure the wrath of God and survive it. Second, because the divinity of the Mediator makes His work infinitely valuable.
Third, see Ursinus on this.
Why does the divinity of the Mediator’s person make his work infinitely valuable?
By placing this in the context of Old Testament worship, we can better understand it. When a man in Jewish society committed some sin, he was required to bring the prescribed sin offering. When he did this according to the stipulations given (Lev 4), that sacrifice was then sufficient to atone for his sin. It was sufficient for him; it was not sufficient to atone for the sins of anyone else. The sacrifice of Jesus was different in this respect. Since Jesus’ sacrifice involved the death of Jesus Himself and He was divine, His sacrifice was infinitely sufficient to atone for all the sins of anyone who was ever born or would be born. Otherwise, we are driven to adopt the so much for so many understanding of Jesus’ death.
What is the so much for so many view?
First, it would be almost impossible to ever find a theologian who actually adopted this view. Nevertheless, if one denies that the death of Christ was infinitely sufficient, then he is driven to the idea that Jesus died for a select number of people and that if God decided to save more people, Jesus would have had to die again to merit their salvation.