Washed in the Blood of the Lamb
Nothing could seem crazier to someone outside the church than people inside the church saying they want to be "washed in the blood of the lamb". It would have to bring up images from horror movies or at least of ritualistic animal sacrifices.
The phrase "washed in the blood of the lamb" begins in Judaism. Under the agreement (covenant) that God had with his people Israel they needed to make sacrifices when they sinned. There were different kinds of sacrifices, plants, food, and sometimes live animals. These sacrifices would make them clean before God.
As God prepared to make that agreement with Israel, while Israel was still held as slaves in Egypt, each family was required to sacrifice a perfect (flawless) lamb. This sacrifice would protect them from the 10th plague, the angel of death. Anyone who did not have the blood of a lamb on the door posts of their house, would suffer. The angel of death would kill the first born son.
The people of Israel did not wipe the blood on themselves. The meaning of "washed in the blood of the lamb" is symbolic. The lamb was symbolic of the Messiah Jesus who would come many years later. The sacrifice of the lamb was symbolic of the sacrifice that Jesus would make by allowing himself, a God, to be killed by Israel. The blood of the lamb symbolizes the blood of Jesus.
Just as the blood of the animal sacrifices symbolically washed the people clean before God, the sacrifice of Jesus would clean the people before God. In other words the sacrifice would take away their sins. Therefore the phrase "washed in the blood of the lamb" really means that the sacrifice of Jesus washes away our sins
But the sacrifice of Jesus was different from the animal sacrifices. Being God, his sacrifice would be great enough to cover the sins of everyone for all time. As a result there was no longer a need for sacrifices for sin.