I think it’s important to complete the verse. I suspect Paul chose his words carefully.
It comes at the end of a wonderful passage which exhorts the Philippians to have the mind that they have in Jesus. And it ends work out your own salvation with fear and trembling, for God works in you, both to will and to do, for his good pleasure.
The first part leaves us Pelagian or semi-Pelagian and we would deserve a lot of what is said about us. It is God in Christ who saves and through miracle and, I think, sacrament draws us into the new life and sets us to walk in the good works He has prepared for us to walk in.
Each of the seven early churches had their problems.
I am not sure about the Pelagian heresy.
I do know that the heresies are the same over time.