I mean, this is just saying that a person is justified in believing literally ANYTHING about themselves and their identity as long as they outwardly say that they are ascribing their beliefs to their version of God.
Since God is a silent, dormant entity whose whims and beliefs only exist within the minds of each individual person who believes in him. Each person can decide, independently, for themselves what God believes, wants them do do, or approves/disapproves of, and there's no one that can tell them differently because God is the 'ultimate' judge and he's not going to say or do anything supposedly until some future day of judgement.....sometimes people have likeminded ideas of their version of God and form into different groups.....this is why we have thousands of different denominations that all define God a little (or a lot) differently and interpret the words of the Bible differently.
And like I said, no, the Bible is not a good source for ascertaining Gods wants, beliefs, and commands since humanity has never been able to agree on what the correct interpretations or meanings of a vast majority of the book really is. Everyone interprets the Bible differently and nobody can ever prove any interpretation or version to be right or wrong because the books writers, and the only people on the face of the earth who have supposedly ever been able to truly 'know' Gods intentions, died thousands of years ago. (assuming you don't believe in any of the denominations that believe God has had more recent modern day prophets that have spoken for him outside of the Bible.) (and also assuming you aren't in a denomination that believes the modern Christian Bible isn't the full or correct version of Gods word)