Disposition
Since assurance is a state of self-consciousness then, the truth of the gospel presses itself into the lives of individuals each with his or her own history, understanding of the gospel, life context, and psychological makeup. That being the case it may face greater obstacles in some Christians than it does in others. An individual may have quite strong faith, much grace, and rich evidence of fruitful service yet lack full assurance because of natural temperament. We are, after all, physico-psychical unities. A melancholic disposition de facto creates obstacles to the enjoyment of assurance—partly because it creates obstacles to the enjoyment of everything.
Sinclair Ferguson, The Whole Christ: Legalism, Antinomianism, and Gospel Assurance: Why the Marrow Controversy Still Matters (Wheaton, Illinois: Crossway, 2016), 219-220