Creationism

[GrandizerII]

Please tell us the reasons you reached this conclusion.

The perceived motivation of the person making the argument is not a reason to call the argument flawed. That would be an ad hominem fallacy. Even badly motivated people may make logical arguments which are sound and valid. 

The argument states that everything contingent depends for its existence on the existence of something else. For anything at all to exist, there must be existence. Therefore existence itself is not contingent on anything else. Therefore existence itself is the first cause. 

The parts that are specific to a given religion require additional arguments, and are not included in the first cause argument, so please show the logical flaw you perceive in only the argument as stated.

Flawed because he inserted god into the argument where a god was not needed for the premise to be understood. He inserted god because he had an agenda to attempt to justify the existence of god to the non religious.

A justification for believers was not necessary. (I think good ol Tom indicated this himself)

https://en.wikipedia.org/wiki/Cosmologic...rarguments

Or are you saying that Thomas did not insert god as the contingent/cause?

Using the word "insert" isn't fair here. Aquinas' arguments lead to the conclusion that the ground of existence exists, and he believed that is equivalent to the God that he believed in. But he didn't shoehorn God in, it's concluded, not asserted.