Formal:
* Sincere: This emphasizes genuineness and authenticity.
* Serious: This focuses on the gravity and importance of the matter.
* Solemn: This suggests a deep seriousness and formality.
* Devoted: This highlights commitment and passion.
* Zealous: This emphasizes strong enthusiasm and fervor.
Informal:
* Serious: A more casual version of "serious"
* Genuine: Emphasizes authenticity and sincerity.
* Heartfelt: This suggests emotional depth and sincerity.
* Wholehearted: This emphasizes complete dedication and commitment.
More specific:
* Intent: This suggests a strong determination and purpose.
* Fervent: This suggests passionate and intense enthusiasm.
* Passionate: This emphasizes strong feelings and emotions.
* Dedicated: This emphasizes commitment and devotion.
* Determined: This highlights unwavering resolve and purpose.
The best synonym for "earnest" will depend on the specific context and the specific meaning you want to convey.