infallible

To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God:

It is by this means, by taking up into itself the noblest elements, that the soul is established in purity, and having fixed its aim on piety, the noblest goal of all, it uses this as its infallible guide and so acquires a definite purpose.