Why do religious people claim no one can fully know Gods truth?

When in fact God revealed it in his Son. I believe those that say you can't fully understand Gods truth are probably cults. No one can understand everything about God. But God has revealed everything we need to know. This is his absolute truth. Jesus said I am the wayN the truth, and the life.That is Gods absolute truth . So why do you deny its impossibe to know? " Now what Im commanding you today is not too difficult for you or beyond your reach. It is not up in heaven so that you have to ask who will ascend into heaven to get it and proclaim it to us so we may obey it. Nor is it beyond the sea so that you have to ask who will cross the sea to get it and proclaim it to us so we may obey it. No the word is very near you: it is in your mouth and in your heart so you may obey it."