In Variable selection method, should we include all the variables?

Question

I've been working on a class project.

While building a model, We've selected a few initial predictors (8 predictors out of 20) based on Business Knowledge.

Next, we wanted to choose predictors based on variable selection methods (Step-wise).

In Variable selection method should we include all the variables or are the Business logic predictors are fine?

As everyone is just studying we're confused on this.