What does John 1:36 mean?

John the Baptist referred to Jesus as "the Lamb of God" (John 1:29). This isn't a reference to a temple sacrifice, but to the days of Passover and Moses. It represents a change in the Baptist's ministry. Up to this point, he had been telling people that the Messiah was coming (Matthew 3:1–3). Now, he tells people that Messiah is here!

The final plague of Egypt was the death of the firstborn (Exodus 11:4–6). Only those whose doors were covered with the blood of a lamb were spared. The preparation for sudden travel and the requirement of blood made a powerful symbol of Jesus' future work on the cross (Exodus 12:5–7, 11). After the judgment, those who stepped through that door were freed from their former slavery (Exodus 12:41).

The instructions given to Moses involved dipping a spongy plant into blood collected at the base of the door. This was then to be touched—or possibly "struck"—on the overhead beam and the two side posts. Whether an explicit goal of the process or not, that would have created a bloody cross on every Israelite door (Exodus 12:21–22).

Everything involving Passover is a metaphor for salvation in Christ. Only those covered by the blood are spared from judgment (Exodus 12:23). Only by going through that door can a person be set free.