I think we come to Christ through faith - based wholly on the finished work of the cross - and we are then to walk in newness of life - with this as our foundation. What matters now is how we build. We are to walk in the spirit and no longer in the flesh but rather denying self. So begins the lifelong process of sanctification. For me, to walk worthy of Him is to walk with Him, actively growing in him using the tools he has given us in his word - not going back to our old dead lives and old ways of doing things. We are saved and sanctified by faith and not any works which we can muster up in our own strength from our own resources.
So a picture of not walking worthy of Him would be turning away from him or seeking to serve him with religious dead works.
As to strongholds - I believe these come down as we walk with God, are washed and renewed through being in the word and as we obey him. I don’t believe a Christian can be possessed. But the work of this fallen world including demonic influence is something we must each daily overcome. In my own life, some of these strongholds broke off very quickly, but others are so strong and so deep, it’s taking a lot longer to be renewed and changed into His image for them to be pulled down. I suspect some won’t be completely removed from me this side of glory. But I’m freer than I once was.