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). Therefore, the Roman Catholic differentiation between venial and mortal sins is an unbiblical one. Sin as a whole separates mankind from God. Sin by its very nature is fatal to us.
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 will lose his or her spouse and potentially the entire family. But there are no distinction between mortal and venial sins.
Once a man has placed 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. The Lord does chasten those whom He loves. Moreover, the grace of God teaches believers to resist sin (Titus 2:11-14). Another dilemma for the Roman Catholic concept is that if we view most of our sin as being "venial" in nature, then we will essentially minimize or overlook our sin nature and the need for God's continual providing of grace.