The two most important, as I see it, are the nature of God and the qualities of true charity. The sense of sin is a function of our concept of God. If we conceive of God as the all-holy One, the God who vindicates his honor by insisting on repentance and obedience to the point of implementing his plan of salvation by the painful death of his beloved Son, we cannot but have a horror of sin and consider it the very worst thing that can befall anyone. All the saints understood this. P guy was moved by the story of Saint Louis of France who, when asked once whether he would commit a mortal sin to avoid getting leprosy, replied that mortal sin was far worse than leprosy and he would far rather contract this disease than be so unfortunate as to commit a single sin.
Reason is a feeble support when passion, whether fear or desire, waxes strong.
The sense of God's holiness and his intrinsic rights over our whole being remain even when the feeling for his love and protection are temporarily in abeyance. Knowing God as the transcendent Creator of all, with such interest in us as requires us to act worthily so as to deserve his approbation is at such times the strongest support of our weakness. Those who truly know God found even the smallest sin, even when not fully deliberate, to be an intolerable burden and hastened regularly to seek his pardon by confession and penance. The other issue raised in this connection is the true nature of charity. This is a vast question.