When we are crucified with Christ by faith in Him, we are to completely surrender every selfish desire and ambition to the perfect will of God. Self-promotion and self-pleasing desires are the greatest obstacle between man and salvation because it is the nature of man to want to please himself, and this self-recognition is a building block for sin. All sin erupts from the desire to please self, and this is what must be crucified with Christ in surrender to the will of God before salvation can bear fruit. This, as Paul attests, is an ongoing process because the spirit, which has been crucified with Christ, still resides within in the flesh, which is still very much alive Romans 7: 18-25.
Many Christians want to be obedient but continue to stumble in sin because they try to hang on to parts of the old self they think they are able to control. This is a self-defeating rationale that relies on the idea that man saves himself by his own works. The Bible teaches us that nothing a man does can save his soul, and salvation is only by the grace of God Eph 2:8-9. Through this gift of grace, man can be born again and be given a new nature to replace the old, sinful self. He must be born again if he is ever to see the kingdom of God John 3:3, and if he is to be born again, his sinful nature must be crucified with Christ, and he must be filled with the Holy Spirit of God.