Baha’u’llah identifies “the king of all acts”

The first and foremost duty prescribed unto men, next to the recognition of Him Who is the Eternal Truth, is the duty of steadfastness in His Cause. Cleave thou unto it, and be of them whose minds are firmly fixed and grounded in God. No act, however meritorious, did or can ever compare unto it. It is the king of all acts, and to this thy Lord, the All-Highest, the Most Powerful, will testify.... (Baha'u'llah, Gleanings from the Writings of Baha'u'llah)