Some aspects of God's will we know from nature, like the natural law (lying is wrong, murder is wrong, etc.).

Other matters depend on God's free choice. To know God's will in those matters, he has to tell us. We know Christ wants us to baptize people because he told us.

Most of the time, our own decision-making process involves discernment using the virtue of prudence and the gift of counsel, not explicit divine revelation. God doesn't tell me whether to have ham for lunch instead of turkey.

This often applies even to very large, very important decisions: Should I marry Susie or not? Should I take that job and move? God doesn't reveal answers to these sorts of things typically. We use prudence and make a decision.

To seek undue certainty about the future or the hidden things of God, things he hasn't revealed, is divination. To make assertions about what is or isn't God's will in respect to contingent events can be dangerous--appointing oneself as God's prophet and mouthpiece.

This is an affront to God's sovereignty, as if God cannot act or refrain from acting as he chooses. It also often belies a lack of trust, as if we don't expect God to do anything and instead invoke him like some kind of mascot or talisman.

The cases where we can speak for God are the cases he has given us, written in the book of nature or the book of revealed truth, entrusted to the Church and especially to those whom Christ appointed to teach on his behalf.

Whenever we begin to make some assertion about what God is going to do, we should be very careful to consider how we know that. What is the source of our certainty? Are we speaking on God's behalf according to his wishes, or using him as a prop for our own purposes?