See verbs/fia, verbs/fía, from (preposition), Jehovah, all, heart, and (conjunction), no (negation), te (pronoun), verbs/supports, in (preposition), tu (adjective), proper, prudence.
It means that you trust the Lord fully, with a great faith from your heart, and do not lean on what you know and have heard, because the human mind in the spiritual is wrong.