...sharper than the sword and finer than a hair

Take thou good heed that ye may all, under the leadership of Him Who is the Source of Divine Guidance, be enabled to direct thy steps aright upon the Bridge, which is sharper than the sword and finer than a hair, so that perchance the things which from the beginning of thy life till the end thou hast performed for the love of God, may not, all at once and unrealized by thyself, be turned to acts not acceptable in the sight of God. Verily God guideth whom He will into the path of absolute certitude.

The Bab, Selections from the Writings of the Bab, p. 95