Faith Lutheran Church - Symbols and Meaning on Page 12
The Scallop Shell – (at six points around the edge of the
bowl). John is said to have baptized Jesus with water
from a scallop shell and it has been used as a symbol of
baptism since then.
I can find no biblical evidence for the scallop shell to be associated with it but did find sermons where a pastor actually said the following showing that its mostly because the art depicts the shell even though the text shows Paul said Jesus was baptised by immersion.
. All of the earliest art that I have seen shows Jesus and John standing in knee deep water with John pouring water over Jesus' head from a scallop shell. Did you know that that shell is the Christian symbol for baptism?
I have to admit that the symbolism of baptism by immersion is amazing. In Romans 6:8 we hear Paul tell us that if we died with Christ, we have faith that we will also live with him. When we are baptized by immersion we enter into the world of water where we cannot survive for long. When we are raised from the water and we take that first breath we have a new appreciation for the air we breathe. So dying to Christ and living to Christ have new meaning and significance for us. But that is the symbolism from a human perspective.
The Baptism of Jesus
I find a couple verses that support immersion over pouring as well as the quote by Paul in Romans...
Just as Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove. Mar 1:10
As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. Mat 3:16
Then both Philip and the eunuch went down into the water and Philip baptized him. 39 When they came up out of the water, the Spirit of the Lord suddenly took Philip away, and the eunuch did not see him again, but went on his way rejoicing. Act 8:38-39
These verses lead me to believe that the shell may have originally just been a convienient way to "iconize" or "symbolize" baptism as it would be difficult to create a simple symbol to represent immersion. Also it would have been common that many baptisms took place in desert regions where standing water or rivers were not common so in those areas baptisms likely were "poured" and scallop shells were preserved by the person performing the baptism for continual use. However, the scripture still supports Jesus being baptised by immersion.