We know that all Israel, from infant to adult, was baptized into Moses at the Red Sea (1 Corinthians 10:1ff), being spiritually inducted into what we might call the “body of Moses” (in conjunction with Jude 1:9 and Zechariah 3:2), the Old Testament church; just as we are baptized into the body of Christ. They were not drowned in the waters like Pharaoh and his army, but were sprinkled (Psalm 77:17ff).
There are many other such baptisms. When Jacob and his family re-entered the land after their exile with Laban, he and all his household crossed the river Jabbok (Genesis 32). Another example is Israel’s crossing the Jordan river to enter the land; this was even connected with a circumcision (Joshua 3-5). Baptism is a sign of salvation, resurrection and even ascension (as though passing through the waters above the firmament), while circumcision is a sign of sacrifice and priesthood; these two are joined together in Jesus (the greater Joshua), so that our baptism unites us to his circumcision (Colossians 2:11-12).
We see a double baptism when Absalom attacked David. When David fled Jerusalem, attention is called to the fact that he and all his people crossed the brook Kidron (2 Samuel 15) as they went to the wilderness. Among those who were thus baptized into David’s exile-death are the Philistine convert Ittai (from the city of Gath) and “all his men and all the little ones who were with him” (v. 22). Then, on David’s return into the land, he and all who were with him crossed the Jordan river (2 Samuel 19). This passage indicates that the elders of Judah made a seemingly unnecessary but very symbolic trip across the Jordan in order to bring David back (vv. 15ff), signifying that their own restoration-resurrection depended not only on their repentance but also on their baptism into David and his exile-death and exodus-resurrection.
This is partly what is meant by the author of Hebrews in saying that we should “go to [Jesus] outside the camp and bear the reproach he endured” (13:13). In context, the author is saying that we are freshly joined to Jesus’s death and resurrection when, week by week in worship, we partake of Jesus’s body and blood in the Lord’s supper. But if the Lord’s supper is a weekly renewal of our union with Jesus, then baptism is our initial and definitive union with him, crossing the heavenly waters in a symbolic exile and exodus.
See also Unbelievers.