To Believe and to Obey

The fundamental duty of man is to believe and to observe the ordinances of God. Thus the Bahá'í Faith makes belief itself the attainment of all good, though it is not acceptable to merely believe and not obey. Likewise, it is not acceptable to be the author of every righteous deed if one does not believe:

The first duty prescribed by God for His servants is the recognition of Him Who is the Dayspring of His Revelation and the Fountain of His laws, Who representeth the Godhead in both the Kingdom of His Cause and the world of creation. Whoso achieveth this duty hath attained unto all good; and whoso is deprived thereof hath gone astray, though he be the author of every righteous deed. It behoveth every one who reacheth this most sublime station, this summit of transcendent glory, to observe every ordinance of Him Who is the Desire of the world. These twin duties are inseparable. Neither is acceptable without the other. Thus hath it been decreed by Him Who is the Source of Divine inspiration.

Kitab-i-Aqdas, preamble