Principles for living, practical advice, secrets of victorious living, calls to discipleship, and instruction all fall under the catergory of law, whether they are offered sternly or gently, whether they are God's commands or our own. The point is not to jettison these words, but (a) to make sure they are God's words rather than our own, and (b) to recognize that even when they are God's, they are different from God's word of gospel; the good news that although we have not done any of the things we said we would do, Christ has been made our "righteousness, holiness and redemption" (1 Cor. 1:30). The law tells us what God expects of us; the gospel tells us what God has done for us.
So law and gospel are not inheretly opposed, but when it comes to how we are saved, these two principles could not be more antithetical. And since our faith at every moment is threatened by our natural tendency to be distracted from its object- Christ- we need the gospel placarded before us not just at the beginning but throughout the Chrsitian life. The gospel is for Christians too. We need to be evangelized every week. It is not by following Christ's example but by actually ebing inserted into Christ, clothed with Crsit, united to Christ- as the Spirit creates faith through the gospel- that we are not only justified but sanctified as well.
-Michael Horton, Christless Christianity (pg. 124-125)
