【Infallible】 , 【inerrable】 ,  【inerrant】 ,  【unerring】  are comparable when they mean incapable, or manifesting incapability, of making mistakes or errors.

【Infallible】 occurs in this narrow sense chiefly in reference to something (as a person, institution, or book) that is accepted as the divinely inspired medium for the revelation of moral or spiritual truth.

【Inerrable】  and  【inerrant】  are erudite synonyms of  【infallible】 and may be preferable to the latter when it is desired to avoid connotations associated with the notion of papal infallibility.

【Unerring】 implies inerrancy, but it stresses reliability, sureness, exactness, or accuracy (as of aim or observation).