How do I explain to my non-Catholic friend that not all sin is equal? Show me where I can use bible to justify the concept of Mortal and Venial sin?
I recommend 1 John 5:17
All wrongdoing is sin, but there is sin that is not deadly.
A little more in depth would be Matthew 5:19
Whoever then relaxes (breaks) one of the least of these commandments and teaches men so, shall be called least in the kingdom of heaven; but he who does them and teaches them shall be called great in the kingdom of heaven.
First, note that some commandments are rated as “least” which is impossible if they are all equal. Secondly, the person who breaks one of those “least” commandments is still in the kingdom of heaven. Therefore its logical that there are some sins that don’t bar us from heaven while there are others that do.