First off, you can tell that Peter and John had walked with Jesus because they opened their statement with a question just like Jesus did consistently to the Pharisees, Sadducees, and Pilate. They showed their obedience to Christ by asking, "Should we obey you or God?" If any one answers that question with obeying "themselves", that is probably the most arrogant and blasphemous statement ever because they are literally denying the fact that Jesus Christ even died in their place.

Also, in John 9 when Jesus healed the blind man by rubbing mud and spit on his eyes, the Pharisees threatened to expel the blind man's parents from the synagogue. Undoubtably Peter and John knew that could've happened to them as well. More than anything though, they knew that they could be put to death just from knowing how they crucified Jesus. Even death had no power against the Holy Spirit that filled Peter...WOW.

Acts 4:19