Only one sin. ( Numbers 32 v 23 )

When the Bible tells us that all have sinned and come short of the Glory of God,(Romans 3 v23) that's a fact,how serious does one take that?,and how serious do we take sin?. I heard a person say,as regards something that was being done,'that's not a big sin', I think a lot of people categorise sin ,from a human point ,we see murder ,child abuse,rape as being very severe ,and then other things as not so serious.But how does God look on sin ?, well He takes all sin very seriously,even sins of ignorance.I was talking to  a friend recently,and he mentioned about his mother,whom he loved,she was not saved .When he spoke to her as regards salvation,like many people she said the usual thing ,she went to church,and didn't do very much wrong.Her son said this to her,'one sin will take to you to hell,'he went on to say how he invited his mother to go to church with him,which she did, it was snowing so not many people were there.My friend went and spoke to the preacher before the meeting began,he said to him,that there was only one unsaved person in the meeting,and that was his mother.He said to him that in his message say,'that even one sin will take you to hell',and the preacher did that.At the end of the message he made an appeal for anyone who wanted Jesus as their Saviour to put up their hand,and my friends mother did,she became a Christian, and when she died ,she didn't go to hell, she went to heaven.