It is important to realize that our Lord Jesus Christ perfectly fulfilled the law of God, both in its requirements and its penalty. He did what Adam failed to do render perfect obedience to the law of God. Then by His death He completely paid the penalty of a broken law. So, from the standpoint of obedience to the law and of paying the penalty for breaking the law, He perfectly fulfilled the law of God.
Therefore when God justifies us, or declares us righteous, He does not create some sort of legal fiction, calling something righteous that is not. Rather, He declares us righteous on the basis of the real, accomplished righteousness of Jesus Christ, which is imputed or credited to us through faith.
The typical evangelical paradigm is that the gospel is for unbelievers and the duties of discipleship are for believers. But the gospel is for believers also, and we must pursue holiness, or any other aspect of discipleship, in the atmosphere of the gospel. To do that, however, we must firmly grasp what the gospel is and what it means in practical terms to preach it to ourselves every day.
The single passage in all of the Bible that most clearly and completely explains the gospel is Romans 3:19-26. A minister friend of mine calls this passage "The Heart of the Gospel." So if we are going to preach the gospel to ourselves every day and learn to live by it, we need to understand Romans 3:19-26.