Kalam
His first contribution was to Islamic Theology - Kalam.
It was falling under the influence of Greek Philosophy, tarnishing the reputation and generating negative attitudes towards it.
Al- Ghazali openly criticized philosophers such as Aristotle in his book "the incoherence of the philosophers".
Example Aristotle said that resurrection was a myth, but Ghazali said because Allah is all-powerful, Allah created life and can also re-create it.
Ghazali’s critique was so successful that Greek philosophy never influenced Kalam again.
Sufism
Ghazali became fascinated with Sufism. Sufism focuses on direct personal experiences with Allah as ways of becoming a more dedicated adherent.
A strong connection to the Sufi idea that knowledge could be gained through mystical disciplines, that would bring him closer to Allah.
He then wrote his most famous text 'the revival of religious sciences' which refined and reshaped lots of Islamic teachings.
Education
He used his influence to change people's opinions on education. 'Revival of Religious Experiences', Al-Ghazali emphasizes that education shouldn't be limited, but should be inclusive of all aspects of life.
He also explained the purpose of education and gaining knowledge is to assist man in achieving full happiness by achieving the closest possible relationship with Allah.