Here is where holiness comes in. Holiness is the inevitable mark of the disciple of Jesus Christ. An unholy Christian life is simply an oxymoron, a contradiction in terms, a denial of the reality of justification by faith alone. In Romans 6:12–16, Paul unpacks the significance of a holy life in terms of a radical transition having taken place—from being under the law, here meaning the individual’s being dead unto sin and being at the service of unrighteousness, to being under grace, here meaning the individual’s having become alive unto God and serving the cause of righteousness.
Holiness is the spiritual and practical evidence that transition has taken place and is properly functioning in real terms. Again, Hodge is worth quoting: “Grace, instead of leading to the indulgence of sin, is essential to the exercise of holiness.” Under grace, holiness is the sign of justification’s having occurred. Without the visibility of holiness in the Christian life, all the caricatures of fictional justification and antinomianism unfortunately become possible. An unholy life is an excuse for mockers of the Christian faith to be reinforced in their wrong prejudices against the gospel. A holy life is a sign of the truthfulness of God’s Word and the power of His grace to bring life where death and sin have previously reigned. What a great responsibility upon us disciples of Jesus to be holy, because God is holy (1 Peter 1:16).