Just re-read Bruce Winter's essay in the Brower and Johnson collection Holiness & Ecclesiology which has a lot of really helpful things to say about Paul's teaching on sanctification (an event not a process) and how Paul dealt with the Corinthian's failure to live up to their calling.
In particular he reminds us that one of Paul's major concerns was how his hearers' conduct helped or hindered their neighbours seeing the gospel in action. Commenting on what Paul says about food offered to idols, Winter says: 'He [Paul] rejects the popular aphorism that 'all things are permitted' (10:23), and demands what was antithetical, i.e., they give no offense to Jews or Greeks or to the church of God but seek the welfare of their neighbours that they may be saved.'
That seems spot on to me - and goes a long way to explaining Paul's teaching on a whole range of things in the letter. The church's porous boundaries meant that Christians - individually and collectively - had to have a firm grasp of their identity in Christ and sensitivity to how their conduct commended their faith to outsiders.
They were sanctified, set apart (1:2, 4-7) so that their neighbours would see Jesus' revolution in action through how they lived and related to each other. It was their failure to live up to this calling that exercised the apostle so.