Recently I was trying to show my Catholic mother that the doctrine of purgatory is nowhere supported in scripture. She then proceeded to tell me about mortal sins and venial sins. Are there any scriptures which discredit the idea of mortal and venial sins? If so I would appreciate if you could point me to any.
The idea that there are different degrees of sin is not Biblical. All sins are mortal in that all sins lead to death (Romans 6:23). Neither is there any Biblical support for having some sins forgiven but not others. James 2:10 says whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it. Either all our sins are forgiven or none of them are.
For a born again believer there are no unforgiven sins because Jesus paid the price for all the sins of our life at the cross (Colossians 2:13-14). The moment we accept His death as payment in full for our sins our salvation is guaranteed (Ephes. 1:13-14), because by His one sacrifice He has made us perfect forever (Hebrews 10:14). For these reasons purgatory would serve no purpose and that’s why you can’t find Biblical support for it.