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He mentions that while at Church, they had a sermon called "Jesus in Disguise", which preached that you should be nice to everybody as you never know who is actually Jesus pretending to be someone else.
Greg notes that the sermon is probably supposed to make him want to be a better person, but all it does is make him paranoid of guessing wrong.
He then imagines himself shining Fred's shoes and Fred thanking him, while Greg replies "No problem, 'Fred'."