Although we have lost the familiar vs. formal forms of address in English, I can tell you, as a polyglot, that when God is addressed in the Bible, the form used is the informal one (tu in French, du in German, ты in Russian, etc.). While respect is necessary, God wants us to come to Him as our Father, as one who is close to us and not unapproachably distant, as in Islam. If you read the King James Bible, we address God as Thou, which is actually the informal form ("you" is the formal/plural form). We still capitalize the word to show respect, however. Just like you would capitalize "Dad".