True Christianity for Real Believers
Satan being free during both the Jewish and Christian dispensations is a big part of the reason why those two dispensations fail and end in judgment. Both Israel and Christendom could not make good their corporate responsibility and testimony while the devil was/is not bound. The principle is that as long as the devil is free, mankind will fail. This is a historical fact of Scripture. This principle will again ring true at the end of the millennium when Satan is loosed from his captivity.
More information that addresses the Millennial dispensation and the effect of Satan being loose:
Chapter Twenty-Two – The Importance of Satan being bound for a Thousand Years
https://www.reintgenchristianbooks.com/chapter/chapter-millennial-principles/
https://www.reintgenchristianbooks.com/dispensations-differing-principles/