Bouvier's Law Dictionary, Revised 6th Ed (1856):
ATHEIST. One who denies the existence of God. 2. As atheists have not any religion that can bind their consciences to speak the truth, they are excluded from being witnesses. Bull. N. P. 292; 1 Atk. 40; Gilb. Ev. 129; 1 Phil. Ev. 19. See also, Co. Litt. 6 b.; 2 Inst. 606; 3 Inst. 165; Willes, R. 451 Hawk. B. 2, c. 46, s. 148; 2 Hale's P. C. 279.