In ancient Hebrew culture it is customary to use lambs as atonement for one's sins such as in the story of Abraham and Isaac, when the Lord stopped Abraham from killing Isaac, God provided a ram instead for the offering. Even the high priest who enters the holy of holies has to offer up a sacrifice for his own sins. He cannot do it for another much less for many people like Jesus who took upon Himself the sin of the whole world. It was unheard of at that time.