I've been mulling over the relation between the gospel and holiness. On Sunday I preached an inadequate sermon on the Holy Spirit in Galatians, the main point of which was that Paul really seems to expect that we will be made holy in our actions by the Spirit (not our own efforts), and that we receive the Spirit as we hear the message of Christ crucified and respond in faith. Therefore, the key to practical, lived-out holiness is focussing on and believing the gospel.
I think there is something similar going on in Ephesians 4:20-24. Paul has just told them to change their behaviour - "you must no longer walk as the Gentiles do". Then he refers them back to their experience of hearing the gospel - "assuming that you heard about him and were taught in him, as the truth is in Jesus, to put off your old self... and to put on the new self..."
Three things that I see going on here:
1. They heard about Jesus - that is to say, they heard the message about what happened to Jesus in his death and resurrection. They heard that Jesus truly died, and rose again.
2. They were taught in Jesus - which I take to mean that they were taught about what it means to be in Jesus, to be joined to him in his death and resurrection. In him, they also died and rose.
3. They put off the old self and put on the new - which simply means bringing their behaviour into conformity with what is true about them because of their unity with Jesus in his death and resurrection.
The key, again, is the mind - thinking and believing the gospel. But this is not just CBT. It is not just thinking ourselves into holiness. The foundation of it all is the little phrase "as the truth is in Jesus". This is not sanctification by wishful thinking; it is sanctification by the fact that my old self is really dead, and I have a newly created identity. I am a new man (note that old self/new self is old man/new man in Greek - this is literally the abolition of the person I was and the institution of a whole new person). This has happened to me, because of what has happened to Jesus.
The struggle of sanctification is the struggle to see myself "in Jesus", and therefore as dead and raised again.