Resolving Matters of Sin
Paul introduces us to the problem, not the minutiae and defines the principles to use to resolve the problem (of sin).

v 1 the problem - immorality and expands it telling the reader that if this problem is not taken care of it will pollute the church.

v 2, 6 This church has become proud in themselves and has noted the problem but has ignored it.

v 5 same wording as in 1Tim 1:20

v 7 leaven is indicative of the permeation of sin.

v 9 "I wrote to you 'in epistolary fashion'" (aorist tense) meaning "in this letter" do not associate - do not mingle -

v 10 Paul is not writing (in this case) about unbelievers but...

v 11 ...is talking about a "so-called" believer