So if were a sinner and I went down to local Baptist church and I said I accept Christ and got dunked in water, I could do whatever I wanted to do and it would not be sin, Hey sign me up , cause I'm going to the bank.
If you are truly saved, then you WANT to do the will of God. You are moved by the Holy Spirit to obey God. You are repentant when you sin, and confess before God and man that you have failed.
"even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will," Eph. 1:4-5
This is a verse that corresponds with the closing verses of Romans 8. God choses us, he saves us, he keeps us. That is salvation and sancitification.
As for those who appear to be saved, but then go back to sinning, John says it best.
"They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us." 1 John 2:19