The blood was a sign to the death angel to "pass over" their homes when it went through Egypt. Because of it, Israel's firstborn were saved, while Egypt's firstborn died.
The yearly ritual of Passover represents the death of Jesus Christ, who was God in the flesh. The innocent lamb had to be without blemish because it represented the only Man who ever lived a perfect, sinless life. Jesus Christ was the Lamb of God who gave His life and shed His blood so that we may be saved from eternal death by paying the penalty for our sins. Through faith in His sacrifice, we receive forgiveness of sin and come into a right relationship with God. Because His life was worth more than all human life combined, His sacrifice paid the price for all sin. He redeemed us from the penalty that the breaking of God's law imposes and freed us to live righteously.
Earl L. Henn (1934-1997)
Holy Days: Passover
John the Baptist makes it clear that, though he was a prophet, he was not the Prophet, but rather was sent as a preparer of the way for the One who was the Prophet. John the Baptist's ministry was "unto repentance." He came revealing to people their sins, something the prophets always did. What we have here is not so much his ministry as the unique position he was in, chosen by God to be the one who would introduce the Messiah to the Hebrew world.
John W. Ritenbaugh
Prophets and Prophecy (Part 1)