When you read John 20:19-23 there are a couple of Old Testament stories that come to mind. The next two posts will look at these parallels.
The first is the Cain and Abel story - Genesis 4:1-16.
Notice that the disciples function like Cain in that they participate in the death of the other character - Jesus and Abel.
Then you might notice that both the disciples and Cain are gripped by fear.
You will notice that both stories have the murdered speaking beyond the grave. In the case of Abel his blood cries out to God. While in the case of Jesus he appears in the room the disciples locked themselves into.
Cain and the disciples both receive a 'mark' from God.
The stories are similar in a number of ways, until the way the one who is murdered responds. Abel's blood "cries out" to God - the same way the Israelite cry out to God under Egyptian oppression. When this blood cries out it is "shrieking" and "clamoring" to God. But the blood of Christ does something different - he proclaims peace and forgiveness.
Could it be that John 20 is making reference to the first murder (Cain and Abel) in order to make a distinction that the response of the second murder (the murder of Jesus) is a new commandment?
Are we being invited to consider how we will respond to when someone kills us, stabs us in the back, slanders, destroys, criticizes, diminish, hurts and abandons us? Will we respond like Abel and "cry out" or will we declare peace and forgiveness like Christ?
It seems if we are Christians - rather than "Abelists" - then it is clear what we are to do.