Peter brings up an excellent point that is easy to see in the second half of verse two: "by reason of whom the way of truth shall be evil spoken of". Think of the stigma on church. Think of the reasons people choose not to go, and why those that blatantly oppose it give reason to trash it. It is because of these false teachers. If a Christian is truly following in the way of truth, some may speak badly of him, but it is without reason and will not stand. But if ungodly men and false teachers and playing the part and are fakes, that makes them hypocrites. This gives the world the ability (truthfully though unwarranted) to say that churches are full of hypocrites. They look past their own faults and point out others', while ignoring the fact that there are genuine people as well. But it is these certain men that allow the way of truth to be "blasphemed" (Num. 22-23, though mentioned later; Acts 1:18, 5:1-11). They open up the door for stereotypes and badmouthing from wicked people. Therefore another guilt of these ungodly men is bringing reproach on the church of Christ.
The application: Those who are legitimate must separate themselves from hypocrisy and not waste time in frivolous arguments, but rather contend for the faith and let the Holy Spirit do the reckoning through them.