We can only know that which God has chosen to reveal. This is the preciousness of the Bible: it is the record of His disclosures and His revelation to man.
“The secret things belong to the Lord our God, but the things revealed belong to us and to our sons forever, that we may observe all the words of this law” (Deuteronomy 29:29).
“Therefore everyone who hears these words of Mine and acts on them, may be compared to a wise man who built his house on the rock” (Matthew 7:24).
“How blessed are those who observe His testimonies,
Who seek Him with all their heart!” (Psalm 119:2).
(Spotlight 7, Lesson 1 in Doctrine 101: Learning about God)