From James Buchanan's classic book:
The best preparation for the study of this doctrine is--neither great intellectual ability, nor much scholastic learning,--but a conscience impressed with a sense of our actual condition as sinners in the sight of God.
A deep conviction of sin is the one thing needful in such an inquiry,--a conviction of the fact of sin, as an awful reality in our own personal experience,--of the power of sin, as an inveterate evil cleaving to us continually, and having its roots deep in the innermost recesses of our hearts,--and of the guilt of sin, past as well as present, as an offence against God, which, once committed, can never cease to be true of us individually, and which, however He may be pleased to deal with it, has deserved His wrath and righteous condemnation.
Without some such conviction of sin, we may speculate on this, as on any other, part of the divine truth, and bring all the resources of our intellect and learning to bear upon it, but can have no suitable sense of our actual danger, and no serious desire for deliverance from it.
To study the subject with advantage, we must have a heartfelt interest in it, as one that bears directly on the salvation of our souls; and this interest can only be felt in proportion as we realise our guilt, and misery, and danger, as transgressors of God's Law.
...without some heartfelt conviction of sin, we could have no feeling of personal interest in the doctrine of Justification, such as is necessary to command our serious attention in the study of it, so we should be scarcely capable of understanding, in their full scriptural meaning the terms in which it is proposed to us, or the testimonies by which alone it can be established.
James Buchanan, The Doctrine of Justification, p. 222-3