One of the old-school theological “parts” of sanctification is mortification. We really need to get that word (or at least the concept!) back into our every day Christian speech. Mortification means the killing of the Christian’s sinful nature. The Westminster Standards use the term three times (WCF 6.5, 13.1, & WLC 167); the Heidelberg uses the doctrine/terms of mortification in Lord’s Day 33. This teaching comes from Scripture, of course: “Put to death therefore what is earthly in you” (Col 3.5).
Here’s how Ursinus talks about mortification: it “is not without wrestling and pain” (Commentary on the Heidelberg Catechism, 471). Luther said, “Saints must, by a vigorous and unceasing warfare, subdue their sinful lusts…. The Christian must contend against himself” (Sermons, 4.1.229). Kuyper was brilliant too: “God’s child remains the old man’s grave-digger until the hour of his own departure” (The Holy Spirit, 484).
All this to say: sanctification – growing day by day in grace and knowledge – will hurt. We will be conformed to the image of Christ, but it will take some pain and dying! Figuratively speaking, we may have to cut off a hand or foot (cf. Mk. 9.42-45); we cannot set the shovel down while burying the old man. We may get blisters on our hands from digging so much. We may have to part company with old friends, we may have to actually deny ourselves and take up a heavy cross. We may have to undergo some strict discipline from our Father, which at the moment may be “painful rather than pleasant,” but it is good for us: later it produces the peaceful fruit of righteousness (Heb. 12.11). In Peter’s terms, we may be “grieved by various trials,” but again, it is for our good (1 Pet. 1.6-7).
As a side, I think its interesting to note how churches title themselves “life” or “new” or something like that; after all, who would go to a church that had “death” or “sin” in the title? Yet “new” and “life” is only one side of the coin of sanctification: the other is “put to death your old self!”