“It is popularly held that whereas salvation in the New Testament is obtained through faith, Old Testament saints were saved by fulfilling the law. Close examination of Old Testament texts belies this assumption, however. Actually, the important factor was the covenant established with his people by grace; the law was simply the standard set for those people who would adhere to that covenant.” [p. 988]
“Not only the reception of righteousness but also the continuance of the Christian life is by grace, not by works which fulfill the law. And yet Christians are nonetheless to regard the biblically revealed law as an expression of God’s will for their lives, for, as we have seen, the law has not been abolished. Paul notes that we can fulfill several specific commandments of the law by love (Rom. 13:8-10). He reiterates the importance of the command to love one’s father and mother, which is the first commandment with promise (Eph. 6:2). Thus, Ladd observes, ‘It is clear that the Law continues to be the expression of the will of God for conduct, even for those who are no longer under the Law.’
“It is important to draw a distinction between attempting to observe the principles embodied in the law and legalism. Scripture does not give us any basis for disregarding God’s revealed commands. Jesus said, “If you love me, you will obey what I command” (John 14: 15), and “You are my friends if you do what I command” (John 15:14). We are not at liberty to reject such commands; to do so would be an abuse of Christian freedom. Therefore, we must seek to guide our lives by these precepts. Such behavior is not legalism. Legalism is a slavish following of the law in the belief that one thereby earns merit; it also entails a refusal to go beyond the formal or literal requirements of the law. It is ineffectual because it ignores the facts that we never outgrow the need for divine grace and that the essence of the law is love.” [p. 990]
Erickson, Millard J. Christian Theology. Grand Rapids: Baker Book House, 2001.