“The Pauline doctrine of justification by faith alone, which we have just treated at considerable length, is, as we have seen, the very foundation of Christian liberty. It makes our standing with God dependent not at upon what we have done, but altogether upon what God has done. If our salvation depended upon what we have done, then, according to Paul we should still be bondslaves; we should still be endeavouring feverishly to keep God’s law so well that at the end we might possibly win His favour. It would be a hopeless endeavour because of the deadly guilt of sin; we should be like debtors endeavouring to pay, but in the very effort getting deeper and deeper into debt. But as it is, in accordance with the gospel, God has granted us His favour as an absolutely free gift; He has brought us in right relation to Himself not on the basis of any merit of ours, but altogether on the basis of the merit of Christ. Great is the guilt of our sins; but Christ took it all upon Himself not on the basis of any merit of ours, but altogether on the basis of the merit of Christ. Great is the guilt of our sins; but Christ took it all upon Himself when he died for us on Calvary. We do not need, then, to make ourselves good before we become God’s children; but we can come to God just as we are, all laden with our sins, and be quite certain that the guilt of sin will be removed and that we shall be received. When God looks upon us, to receive us or to cast us off, it is not we that He regards but our great Advocate, Christ Jesus the Lord.
Such is the glorious certainty of the gospel. The salvation of the Christian is certain because it depends altogether upon God; if it depended in slightest measure upon us, the certainty of it would be gone. Hence appears the vital importance of the great Reformation doctrine of justification by faith alone; that doctrine is at the very centre of Christianity. It means that acceptance with God is not something that we earn; it is not something that is subject to the wretched uncertainties of human endeavour; but it is a free gift of God. It may seem strange that we should be received by the holy God as His children; but God has chosen to receive us; it has been done on His responsibility not ours; He has a right to receive whom He will into His presence; and in the mystery of His grace He has chosen to receive us.
That central doctrine of the Christian faith is really presupposed in the whole New Testament; but it is made particularly plain in the Epistles of Paul.” -J. Gresham Machen, “What is Faith? pages 199-201.