"I'm studying the Bible with a friend who is still coming to faith. This question came up: 'Just because God exists doesn’t mean he must be a perfect being, does it? He could be omnipotent, omniscient, and omnipresent without being perfect; those three qualities do not necessitate perfection. So how do we know who he is, apart from what the Bible says?' I could use your guidance on how to tackle that question." -- Andrew (2012)
Sorry to take 4 years to reply, Andrew. There's no end to the Bible questions in my inbox, and as you know we select those not already addressed in the website (Q&As or articles). Still, it can get backed up. Thanks for your patience.
Yours is an interesting question, especially because in religious and philosophical discussion, debate, and argument it is nearly always assumed that God is the maximally perfect being. (Even by atheists.) But I can see the point. If you or I knew everything, and had immediate access to the entire universe, and with unlimited power, it is easy to imagine that we might be tyrants -- without a trace of benevolence. We have only to look at dozens of power-hungry despots, presidents and prime ministers in our world who refuse to step down, skewing ethics and rewriting laws to suit themselves.
You seek evidence outside the Bible for God's perfection. I see at least two areas where we might make some progress with your query. There is (our) experience, and there is logic -- let's go there first.
Your friend thinks the omnipotent / omniscient / omnipresent triad isn't enough. But in the case of God, is that so? First, he is holy: wholly other than us, wholly pure and righteous and holy-- morally perfect. Divine perfection thus indicates not only intellectual perfection, but moral perfection.
Yet a fifth characteristic of Deity, flowing from the others, is omnibenevolence. God always does what is good -- what is best for the greatest number. How could one who knows all and has all power not be all-good? Omniscience enables him to be aware of the lies of the evil one. He cannot be deceived into diminishing his goodness. He is fully aware of our needs, and fully able to meet them. With this knowledge, God cannot be tempted.
Thus he remains good in every possible way. (Note that Satan is neither omniscient nor omnipresent.) An all-powerful, all knowing, all-present, all-loving, and all-holy God would be perfect in every way. (Have I missed anything?)
We could also attempt to answer the question from the angle of our experience, but that’s more difficult, since often the Lord does not explain his ways to us, and our interpretations of his actions may be subjective.