Formal:
* Commitment: A strong promise to do something.
* Devotion: Strong love or loyalty to something.
* Zeal: Great enthusiasm and energy.
* Assiduity: Diligence and persistent effort.
* Assiduousness: The quality of being diligent and persistent.
* Perseverance: Continued effort and determination despite difficulties.
* Resolve: Firm determination to do something.
* Consecration: The act of dedicating something to a special purpose.
Informal:
* Passion: Strong enthusiasm or excitement for something.
* Love: A deep feeling of affection and care.
* Heart and soul: All of one's effort and energy.
* Going the extra mile: Putting in more effort than is required.
* Giving 110%: Working harder than is normally expected.
Other options:
* Loyalty: Faithfulness to a person or cause.
* Fidelity: Faithfulness to a person, promise, or duty.
* Single-mindedness: Focusing all of one's attention and energy on a single goal.
* Tenacity: Persistent determination.
* Grit: Courage and determination to succeed despite difficulties.
The best word to use will depend on the specific context of your writing or speech.