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It depends on how you define sinless perfectionism. First, there are sins that lead unto death and sins that do not lead unto death (1 John 5:16). Second, I am not claiming to have acheived perfection and many other Anti-OSAS proponents will not make that claim, either. The key difference here is that one is not sinning with the thinking they are saved. I am talking about those types of sins that lead unto death such as lying, hating, murdering, lusting, stealing, and getting drunk, etc. Paul lists these sins several times and says at one point (in relation to those sins)..... "be not deceived" ..... "for the unrighteous shall not inherit the Kingdom of God." We are not talking about unintentional sins where a believer might drink socially at a restaurant and make another brother or sister to stumble (without their knowledge that they were being watched).
An obediance born of love changes the heart not counting sins instead of blessing. I obey because I love Him so much, any good thing I do stems from wanting to please Him not worrying about rule keeping.
The focus on doing good like Jesus did, aware everyday He intercedes for me and understand I will fall but extends His Hand of mercy to help me up not beat me down.