As an example of eisegesis I think yours is first rate: the question you claim to answer is whether or not scripture indicates creatio ex nihilo or ex materia, and offer a passage in support of ex nihilo which literally states:
By persuasion [ Πίστει ] we understand that the universe was completed [ κατηρτίσθαι ] by the word of God so that what is perceived [ βλεπόμενον ] came to be [ γεγονέναι ] not from what is shown [ φαινομένων ].
You agree that κατηρτίσθαι refers to completion, but omit the fact that completion denotes prior disorder - ie, that which is incomplete - and not the creation of something new. Note that had that been the intent, there is a perfectly suitable word in Greek to describe such an act, which is κτίζω and which is notably absent from the text. So yes, κατηρτίσθαι does suggest an evolution ( of changing to an ordered state from an unordered state ) in contrast to creation.
The real justification for your interpretation of the passage as supporting ex nihilo creation is simply this:
The context is clearly speaking of creation of the universe by God's spoken word, and the terminology leads us to interpret what he is saying as creation ex nihilo
You justify your opinion of the text, which serves to justify your opinion of creatio ex nihilo, by declaring that the context of the text must mean creatio ex nihilo, and thus that the "terminology" cannot be taken literally, but must figuratively mean creation from nothing.
In other words, conclusion precedes your premises, and justification is circular.