The Decretive Will of God is that which is God’s sovereign will that we may or may not know, depending on whether or not God reveals it to us.  The decretive will is God’s direct will where he causes something to be, he decrees it.  For example, God has caused the universe to exist as well as Christ‘s incarnation.

• Job 23:13, “But He is unique and who can turn Him? And what His soul desires, that He does.”
• Psalm 33:11, “The counsel of the Lord stands forever. The plans of His heart from generation to generation.”
• Isaiah 14:24, “The Lord of hosts has sworn saying, ‘Surely, just as I have intended so it has happened, and just as I have planned so it will stand.'”
• Isaiah 46:10, “Declaring the end from the beginning and from ancient times things which have not been done, ‘Saying, ‘My purpose will be established, and I will accomplish all My good pleasure’”.
• Acts 17:24, “The God who made the world and all things in it, since He is Lord of heaven and earth, does not dwell in temples made with hands.”

See also Preceptive Will (God’s good will for man) and Permissive Will (God permits bad to happen).