になる決心をした

decided to become

→ に
phrase Rare #29,022
decided to become Used after a profession or role to say that someone made the decision to become it. The pattern になる決心をする means “make the decision to become.”
JA: 彼は医者になる決心をした。
EN: He decided to become a doctor.

お前は教員になる決心をしたのか

Have you decided to become a teacher?

彼女は秘書になる決心をした

She decided to become a secretary.

私は医者になる決心をした

I decided to become a doctor.

彼はパイロットになる決心をした

He decided to become a pilot.

どうしてビルエンジニアになる決心をしたのか

Why did Bill decide to become an engineer?