Two Mistakes Christians Make

People tend to make two mistakes when they think about the redeemed life. The first is to underestimate the sin that remains in us; it’s still there and it can still hurt us.
The second is to underestimate the strength of God’s grace; God is determined to make us new.
As a result, all Christians need to say two things.
We admit that we are redeemed SINNERS.
But we also say boldly and joyously that we are REDEEMED sinners.
Cornelius Plantinga Jr., Beyond Doubt (p. 89)

I believe that all Christians make these two mistakes, some err more on one side than the other. If you underestimate the sin that remains in you, than you err on the side of lawlessness or licence. You tend to think you're better than you really are which is rooted in pride, you overestimate your spirituality and began to think that you're grace given gifts are somehow rooted in your own goodness or ability. You think that your gifts mean God is approving of your lifestyle.You justify bad behaviour and play down sin in your life. You slide by on your gifts but your character is a mess.
If on the other hand you underestimate the strength of God's grace you err on the side of legalism. You believe it all depends on you and you live your life constantly trying to earn your way to heaven. You become over zealous and critical of others who don't measure up to your standards. You lack real joy and are generally miserable and you make others miserable. You do not understand the gospel and no one wants the kind of Christianity you offer. You're just like the Pharisees of Jesus day, self righteous people who claimed to know God but couldn't recognize him when he was standing right in front of them.
One thing is true of both errors you are generally unteachable.