Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."

 (a) Logos ( G3056) is the great Stoic word for the reason which guides the universe; logikos ( G3050) is a favourite Stoic word which describes what has to do with this divine reason which is the governor of all things. If this is the word's connection, clearly spiritual is the meaning. 