Flavel teaches another important matter under the subject of mortification of sin, namely on friendship. True friends, needed friends, are friends that reprove one another. He illustrates saying, “Christian, thou knows thou carries gun-powder about thee, desire those that carry fire to keep at a distance from thee; it is a dangerous crisis when a proud heart meets with flattering lips.” His point is that it is a dangerous thing to have friends who only tell you how wonderful you are when there is the reality of sin within. God may use a faithful friend, he says, or some malicious enemy to do it, but you need help to do the work of mortifying sin. He illustrates saying “It is said of Alexander, that he bid a philosopher (who had been long with him) to be gone; for, said he, so long thou has been with me, and never reproved me; which must needs be thy fault; for either thou sawest nothing in me worthy of reproof which argues thy ignorance; or else thou durst not reprove me, which argues thy unfaithfulness.” (For more see sermon 28, in Works Volume 2, p.380)
Christians need faithful friends who help them in the mortification of indwelling sin in this life. According to Flavel, indwelling sin is more dangerous to the Christian than the devil and the world because the devil and the world may quit, but indwelling sin never does.