Not all sins are the same, nor affect the believer, and God’s grace in them, the same. Exploring the reformed material and distinctions on this new page of resources will help you to become much more acquainted with the nuances of our Lord’s will in these things.
Specifically, the Romanists distinguished between venial (light) sins, which need not to be forgiven, and mortal (deadly) sins, which extinguish the work of grace in the Christian. While the reformed outright rejected the distinction in the Romanist sense, as all sins are worthy of condemnation, yet they not infrequently used the distinction nonetheless in Biblical and reformed senses.
A big thanks to Rev. Andrés Eduardo Garcia Gonzalez of Venezuela who provided most of the materials on the page.