Who was Paul referring to as the "many" who corrupt the word of God?
Paul was referring to false teachers—the "many" who adulterate and hawk the Word of God as a watered-down product. In defending his apostleship, he contrasted himself with these corrupt hucksters, saying he came in sincerity, as of God, in the sight of God, speaking in Christ.
Les explains that Paul was defending his apostleship and distinguishing himself from false teachers. In 2 Corinthians 2:17, Paul says he did not come with a product likened unto wine watered down with water, nor like a huckster hawking wares not worth half of what he was charging. All of that is wrapped up in the word "corrupt." Paul says a lot of people do hawk an adulterated product of the spiritual, but he came in complete sincerity. Les imagines these are the kind of people Paul had in mind—false teachers—and points to Jude as probably as good a description as you can find anywhere in Scripture concerning false teachers.