In Mark 12, 1-12, Jesus tells the parable of the farm owner and his tenants.  The parable is a familiar one: the owner lets out his farm to unscrupulous men who beat and kill the servants sent to collect the rent.  Finally, they kill the Farmer's oldest son, in the foolish belief that lack of an heir will result in their inheriting the farm for themselves.  It is clear that the farmer in this parable is God the Father and that his son is The Lord Jesus.  But who are the servants?  Rabanus Maurus (d. 856), a Frankish Benedictine monk who became the Archbishop of Mainz, made the following comments:

" 'He [the owner] sent his servants to the farmers in order to receive their fruit, and when his servants had been seized, the farmers beat one, killed another, and stoned a third.'  It is understood that the first servants whom he sent out were the lawgiver Moses, and Aaron, the first priest of God.  It is written of them, 'He sent Moses, his servant, and Aaron, whom he had chosen' (Psalm 104, 26).  For forty continuous years they sought, from those laborers, some fruit of the law which God had given, but they struck them down with the whip of their tongue and sent them away empty-handed.  'They angered Moses in their camps, and Aaron, the holy one of The Lord.  Moses was troubled because of them, and they exasperated his spirit' (Psalm 105, 16 and 32)."

Rabanus Maurus identifies another servant as King David, and the rest as the various prophets.  

We might ask why these servants, and then, at last, the son, went to their certain harm at the hands of these violent men.  The answer is almost too simple for us: they went because they were ordered to do so by the one who was their master, and in the final case, by the one who was his father.