"But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His." Romans 8.9Being in the flesh is to occupy your thoughts with the things of this world.
Being in Spirit is to live and think about the things of a High and Holy place. Your thoughts are linked to the Word of God. This is where you receive the power to overcome doubt, fear and resist ideas that are not righteous, pure and healthy.