"....in that day" - is that this period in history? what does it mean to not be "lawless"? How can we prophesy, expel demons and perform powerful works in a LAWFUL way?
Zephaniah tells us how. We must observe righteous decrees, seek righteousness.
I encourage you to visit some local Baha'is and connect with them (connecting with others striving to serve God is righteous after all isn't it?) and seek to observe what works they do, the spirit they do it in and see if they are lawful or lawless, and observe if they observe righteous decrees, are meek, or not.
