1. Theological language must use analogy. We can only comprehend the unknown in terms of the known.
2. Theological analogy only works in one direction. If I should say "God is my rock" (2 Sam 22:3), I am suggesting that certain characteristics of a rock (perhaps dependability, solidity, immovability, etc) correspond with similar characteristics of God. I can look at a large rock, a known object, and get some sense of what God is in some ways like.
3. There are no rules as to which characteristics of the rock I should attribute to God, although sometimes a particular text will give us some guidance, e.g. in 2 Sam 22:3 it is as a place of refuge that God is like a rock. However, by using other anologies I will notice areas of overlap and difference which will help me, usually intuitively, to understand what characteristics the analogy is intended to convey. For example, if God is my rock but also my shepherd (Psalm 23) or a mother bird (Deut 32:11), I know that the cold uncaring nature of rocks is not to be attributed to God because other biblical analogies would contradict this.
4. In no way can I then suggest that I can discern information about rocks by looking at God, just because "God is my rock." I cannot look directly at God, hence the need to use analogy to describe what God is like. But even if I could the analogy is only intended to work in one direction, it is not made to inform me about rocks but about God, the assumption of the analogy is that rocks are known and God is not. Just because "God is my rock" it does not mean a rock is divine or merciful.
5. This may all seem very obvious but you would be surprised how often peope who should know better try to drive the analogy train down the street the wrong way. This post will provide a handy link next time I need to point such misdemeanors out.
Let me know what you think. :-)