Basically, they know deep down they are not perfect, but believe they only commit minor sins, so they will be OK.
As long as they can avoid the 'taboo sins' God will in no wise cast them out, they will attain Heaven by succeeding in living a good enough life.
They know, that in their mind they sin, but this is acceptable for they do not fail according to where they have set the bar if you like
ie
They don't watch pornographic films, read pornographic literature, have affairs, get drunk, smoke or use foul language.
God will honour that and turn a blind eye to the rest/excuse it, because they are nowhere near as sinful as Joe bloggs down the road who professes to be a Christian