If you believe that you are part light and part darkness, part saint and part sinner, you will live in a very mediocre manner with little to distinguish you from the non-Christian. You may confess your proneness to sin and strive to do better, but you will live a continually defeated life because you perceive yourself to be only a sinner saved by grace who is hanging on until the rapture. Satan knows he can do nothing about who you really are, but if he can get you to believe you are no different from the natural person, then you will behave no differently from the natural person.
Why does this profile describe so many Christians? Because we are ignorant of our true identity in Christ. God's work of atonement in changing sinners to saints is His greatest accomplishment on earth. The inner change, justification, is effected at the moment of salvation. The outer change in the believer's daily walk, sanctification, continues throughout life. But the progressive work of sanctification is only fully effective when the radical, inner transformation of justification is realized and appropriated by faith.
"But didn't I read somewhere that Paul referred to himself as the chief of sinners?" you may wonder. Yes, but he was referring to his nature before his conversion to Christ (1 Timothy 1:12-16). He made a similar statement of self-depreciation in 1 Corinthians 15:9, but continued by saying: "But by the grace of God I am what I am, and His grace toward me did not prove vain" (verse 10). Paul knew that who he was before Christ and who he became in Christ were two separate identities.
If you claim to be just a sinner, what will you do? You will sin! You are professing that sin is at the core of your identity. That's not what the Bible teaches. Why don't we just believe God that Jesus is at the core of our being and then begin to live like it by His Spirit?