Who is a "model Christian"? A pastor-evangelist who can both disciple a congregation and reach the lost effectively? A nurse putting in long hours, working with compassion and praying silently for her patients?
Peter says: think of a slave. A slave - with no power, suffering for doing good - is a model Christian.
He has already told every believer to “live as God’s slaves” (1 Peter 2v16). Now he writes directly to slaves, and of all the groups addressed in this section on “live such good lives” (2v12 to about 3v13) it is slaves who receive the most instruction.