When men choose to reject the truth, God tends to withhold further truth from them. The result is spiritual and intellectual darkness.
Sin affects men in various ways, but it attacks them in every facet of life. It hinders their walk with God and hurts their ability to reach others for Christ.
Most people view sin as a private or earthy matter. Yet, every sin is against the Lord. In truth, men can sin against the Father, the Son, and the Spirit.
How would making your motive match the Lord's motive change how you approach the Christian life (from witnessing to praying to Bible reading and so on)?