While Scripture does speak of different degrees of sin, the payment for all is the same, spiritual death (Genesis 2:16-17; Ezekiel 18:4; 20; Romans 6:23). Even our smallest violations of God's Law are acts of treason against Him (Matthew 5:21-22; James 2:10-11). He is perfect and holy. He is true to Himself. We have all fallen short of the glory of God (Romans 3:23), and so have incurred the sentence of spiritual death. In judging us, God literally takes into account every spec of our lives (Matthew 12:36).
All sin is paid for by the same shed blood of Jesus Christ (Hebrews 10:10-14; 1 Peter 3:18). Sin does have both spiritual and earthly consequences. One may suffer a loss of rewards in eternity, but remains saved. A murderer suffers capital punishment. A thief receives a jail sentence. An adulterer can face separation from his or her spouse and potentially the entire family. But the Roman Catholic distinction between mortal and venial sins is decidedly unscriptural.
When a man places his trust in Jesus Christ as Lord and Savior, he is justified before God and forgiven of all sins. This is not intended to communicate that a Christian stumbling into sin is permissible, when in reality it is not. God chastens those whom He loves (Hebrews 12:6-8). Moreover, His grace instructs us to resist evil (Titus 2:11-14). However, there are no venial and mortal sins. Sin as a category separates man from God. Sin by its very nature is fatal to us. Another problem with the Roman Catholic concept of mortal and venial sins is that if we view most of our sin as being "venial" in nature, then we will essentially minimize its seriousness and the need for God's continual forgiveness.