Understanding and believing justification by faith alone is more than assenting to a doctrine. It is more than right conclusions from word studies, more than exegesis, more than hermeneutics, more than biblical and systematic theology. Believing it for oneself, teaching it to the church, and proclaiming it in the world are no mere intellectual matters. If we understand it aright we will feel ourselves as sinners in the presence of the Holy God with no covering for our sin, shame, and guilt. We will see that our only refuge is Jesus Christ, his blood and righteousness, freely offered to us in the gospel. Our free acceptance by God will not be in ourselves but in him alone.
Our hearts resist submitting to the righteousness of God in Christ. Justification by faith alone is not an easy doctrine to maintain. It never will be.
James Buchanan gave an good indication why this is the case:
Luther knew human nature too well to suppose that the truth could be preserved in its purity without a constant conflict with error; and he predicted more than once the gradual declension even of the Protestant Churches from this fundamental article of faith. He knew that men would grow indifferent to it, in proportion as they became less impressed with a sense of sin, and less alive to the claims of the Law and Justice of God.
He was soon taught by observation of what was passing around him, as well as by his own inward experience, that there are, in the heart of every fallen man, two great tendencies,--pointing apparently in opposite directions, but equally at variance with the doctrine which he taught,--the one, a tendency to Legalism, or self-righteous confidence; the other, a tendency to Licence, and Antinomian error.
Between these two extreme tendencies, the true doctrine of Justification has often been, as Tertullian said, 'like Christ crucified between two thieves:' and all the errors which have arisen on that subject in the Church, may be ascribed to the one or the other, more or less fully developed.
James Buchanan, The Doctrine of Justification, p. 153-4