"what is the observational phenomenom that makes evolutionism a credible hypothesis ?."

if you really want to understand, then I suggest you get rid of the term "evolutionism" which may have been useful in the 1800s but is worthless today. One thing about science... it moves on, based on new information that helps explain observable facts.

If you want to know what is going on today, you need to use the phrase "modern evolutionary synthesis" which is seen as the best explanation of current data.

Things that were unknown in the mid 1800s, such as nuclear decay, plate tectonics, molecular genetics have been merged with new paleontological finds around the globe - among other sources of information- and these widely disperate disciplines have supported the theory of evolution.

Scientific method is asking questions that provide information used to ask better questions. The answers are used to formulate a better understanding of the world/universe. The goal is to increase our overall knowledge and understanding, not to cling to an ignorant past.

If you really believe in an all powerful, all knowing God... why do you then insist that said being is restricted to only what you can "understand". Sounds like the sin of pride to me.