Q. It is impossible to increase muscle strength and mass by doing pushups? I think we can develop good pecs by doing pushups?
A. Not entirely impossible, but certainly inefficient and largely ineffective.
I still maintain that, even as a beginner, pushups are not the most efficient or effective way to develop your chest. I certainly understand the benefits of doing pushups in a case where you have no weights available and you need to work your chest (or in order to get the blood flowing regardless of whether or not your intention is to train your chest).
Put simply, pushups = good
Bench press = better
Keep in mind, that strength/mass gains appear most quickly for people who are just starting to train. To that end, for a beginner, literally any exercise will give you significant gains.