Logically valid deductive argument is defined like this. If all the premises were true, then the conclusion has to be true. It is important to note here that it doesn’t matter whether the premises are actually true or false. With ridiculously false premises, deductive argument can still be perfectly valid.
Aha! Even perfectly valid deductive arguments do not guarantee the truth of the conclusion. It only guarantees the truth of conclusion only if the premises were true. The conclusion derived by premises must be true if and only if the argument is both valid and sound.
When Greek philosophers reason and argue, their arguments can be perfectly valid. They have good tools of reasoning. In many cases where their premises are also true, their conclusions are true too. With this tool of logic, philosophers could figure out many truths concerning natural world.
However, when it comes to spiritual world and theology, special revelation is necessay for us to know who God is. If God does not reveal himself, our premises concerning God cannot be true.