The mind of Christ is an active mind.  A passive mind is a consciousness not directed by its owner but open to allow any thought to pass through.  It does not accept completely the Word of God as truth.  Therefore, it's always questioning and wondering, which weakens the will, causing the person to be slow to obey God.  Being passive means you are allowing the enemy to come at you at any time and you are not actively guarding yourself.  If the enemy can trick you into being passive, you are not a threat to him; he is not worried about you.  You, on the other mind, have stepped into danger.