Mein Vater hat mich zu dem gemacht, der ich bin.

English Translation

My father has made me what I am.

Why is “dem” there for? why wouldn’t it be “was” somewhere in the sentence?


Welcome to the forum!
The sentence is not easy, so your question is completely understandable.
It is “jemanden zu etwas” machen. “Etwas” is the object, so it has to be Dativ.
If the speaker is a man, he, as the object is “dem”.
“Mein Vater hat mich zu dem (Mann, Menschen) gemacht, der ich bin.”
A woman would say
“Mein Vater hat mich zu der (Frau) gemacht, die ich bin.”

You can have a “was” in the sentence, but only a relative pronoun, relating to a neutrum.
“ich bin ein Wrack. Und mein Vater hat mich zu dem (Wrack) gemacht, was/das ich bin.”