This is a little philosophical and theological exercise. How would you answer the following question:
Does God say He is good because He is good, or is God good because He says He is?
In other words, is God good when measured against some kind of standard of goodness, or does God appear to be good because He is the definer of goodness?
How you answer that question will not only reveal your understanding of "goodness" but also your understanding of God and the bible, His word.
Here's my take.
Christians understand and believe God is good. We believe God is good because the bible says so. And since the bible is God's word, His revelation of Himself, we trust what He says about Himself. Once we have met His Son, Jesus as Christ, our Lord and Saviour, we cannot help but trust what He said. And He said the bible is absolutely trustworthy.
So, the bible, God's word tells us that He is good.
Now, however, when He tells us He is good, do I understand it as God meeting certain criteria of being good, hence He can say that He is good? In other words, is there a kind of standard with which we can (and God can) measure God to see if He is good or bad?
Three options I think we have.
1) There is no standard to measure what is good and evil.
2) There is a standard and God does not meet that standard.
3) There is a standard and God does meet it.
Option 1 is invalid since to choose that leads us to conclude that there is no meaning of good and evil. The bible is babbling meaningless words when it says God is good, since if there is no standard with which we (or someone, even God Himself) can judge what is good and evil, how can we even have the category of goodness? It's like saying God is thick or round, but there is no category for measuring God's thickness or round-ness. It is an absurd position. So we reject that option.
Option 2 means that God is not good since God fails to meet the standard of goodness. It is simply contrary to what the bible says. Hence, we reject this option also.
Option 3 is the only valid one we can accept, if the bible is reliable. There must be a standard of goodness, and God meets that. Should He failed to meet the standard, He would not be good, and the bible would be misleading us by telling us that He is good. But the bible is trustworthy and tells us truth about God. So, there is a standard of goodness, and God does measure up to it.
This lets me answer the first part of the initial question in affirmative. God says He is good because He really is good.
Now, then, if God is saying He is good because He actually is good, must we reject the idea that God is good because He says He is? The phrase does appear to suggest that God could be an evil despot but tricks us into believing He is good by telling us with almighty force that He is good? Or can we still say that He is good because He says He is without harbouring some kind of suspicion that God could be lying to us about His character?
I believe we can. I believe we can say that God is good because He says He is good. Not only that, there is a sense in which we must insist upon.
Let me explain.
Earlier, we established that since the bible is trustworthy, and the words like good and evil are not meaningless but meaningful, it is necessary that there is a kind of standard of goodness and God fulfils that standard. But what is this standard? Or, rather more importantly for my current arguments, where does this standard come from? Is the goodness defined outside God? Is the standard of goodness external to God? Then He is judged by an external authority to be announced good. God could have all the power and strength in the world, but His moral standard, His goodness cannot be sustained by Himself. This is unbiblical idea. God is the creator of all things, visible and invisible. All authority is of Him and from Him. The standard of goodness must be internal to God. God defines what is good and evil. He reveals this through His word and supremely in His Son. We are all made in His image, so we can make sense of goodness that God reveals, and even if one has never been taught about the bible, in general, he can agree with what good is when shown to him.
So there is a sense where we can say that God is good because He says He is. If you cannot agree with that statement, it reveals something about your trust (or mistrust) in His revelation of Himself through the bible. We can validate whether He is actually good or not by measuring His actions against His words and we see the supreme vindication of all this exercise in His Son. In Jesus, His Word became flesh, and showed to us that God is good by His own supreme, excellent, and absolute standard He has Himself defined.
The important point of all this is that all these arguments I presented hinge upon the reliability, sufficiency, and authority of the bible as God's word. Without God's revelation of Himself by His word, the bible, we can only have speculations about God's character and even if you may affirm all the right things about God, you do not have a solid foundation to stand upon.