Error in our Faith

"To believe in vain, then, is to hold, along with the truth, some error which undermines it, makes it void and of no effect. And the same Apostles shows us at large, in his epistle to the Galatians, that however zealous Christians we may be, if we add to Christ's death any requistic whatsoever in the matter of acceptance with God, Christ shall profit us nothing, Christ is become of no effect unto us. In general, the apostles ascribe every opinion or practice which they condemn to some error in faith, or a lie held in the place of the truth, 1 John i,8, and ii,4." -Robert Sandeman, Letters on Theron and Aspasio