I too believe sanctification is immediate upon regeneration. But that perfect sanctification, given to us via the Grace of God, is in us but does not necessarily permeate every aspect of our lives. I believe that is what Paul meant when he said "work out your own salvation." He was saying "externalize (work out) that perfect salvation that God has put within you so that all may see God glorified in your life."Synergistic sanctification... sounds like an oxymoron. I believe in Gospel-centered sanctification which means sanctification happens the same way as justification, by grace through faith in the work of Christ.