"Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage." (Galatians 5:1 KJV) .
"It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery." (Galatians 5:1 NIV)
Freedom, salvation and deliverance is not a once-done, always-done kind of thing. It requires maintenance. Once Jesus sets you free, you don't go back and forget where you used to be, and become careless. Carelessness, negligence and ignorance will get you back into bondage.
The devil has been defeated, but it's not yet his final defeat. His final defeat will happen at the second coming of Jesus Christ. For now, the devil still fights back, and is able to bring back into captivity those who may have been set free especially if such people are careless and ignorant.
The way to maintain your freedom is to stand your ground. The New International Version (NIV) says "Stand firm...and do not let yourselves be burdened again.." This is something you have to do for yourself. How do you stand firm? Through prayers! With prayers reaffirm your victory over the devil everyday.