One is not regenerated when they believe. They must be baptized in water and receive the Holy Ghost, which occurs later in time, no matter how little of time this may be. They are doctrines which explain various aspects of that which happened to us at salvation, which is one event. One believes, repents, is baptized, and is filled with the Holy Ghost. The Bible does not view it as a process that one completes as though we are on some conveyor belt in an assembly factory.
Conversion is an event, but an event consisting of various elements.
It is a one package deal, and the writers of the NT only explained the different aspects of what occurred to us at our conversion. I say all of this to say that those to whom Paul wrote were already converted. They were justified, sanctified, adopted, and regenerated. He was not addressing people who had only been justified. The Bible does not speak of believers like this.