Here is what I mean. Man's sufficiency is to get with it and make it happen and do all the things by himself. We are all familiar with the kingdom of man, but the kingdom of heaven is different.
The Kingdom of heaven makes us understand that we are to depend upon God and his sufficiency which is fully sufficient versus man's sufficiency which isn't sufficient. Why?
Because when we try to live by our own sufficiency, we fail to understand God's truth. However, if we live by His suffciency we are fully sufficient in Christ.
More in the link below.
2 Corinthians 3:5-6 NIV
 Not that we are competent in ourselves to claim anything for ourselves, but our competence comes from God.  He has made us competent as ministers of a new covenant---not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.