The central message of this sermon is that even after finding the truth and becoming a Christian, believers can still struggle with wandering from God's path due to the presence of their old sin nature, referred to as "the flesh." This aspect of our humanity remains with us until we are fully glorified in heaven, and it constantly conflicts with the Spirit within us. The speaker argues that this is not a matter of choice or willpower, but rather an inherent part of our ongoing Christian experience. By acknowledging and understanding this reality, believers can better navigate their struggles and learn to live in harmony with the Spirit, rather than being controlled by their fleshly desires.