"Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak." Matthew 26:41
Of course Jesus is speaking to those who believe that Jesus is the Son of God. The Spirit of God within us is the person of the Holy Spirit and dwells in the believer to guide them and direct them into holy living. Our spirit is that part of us that is invisible to our physical eyes and that part of us that continues on when our body dies. Our spirit is strengthened when we adhere to the ways of God and weakened as when we do that which we know is not godly. Our spirit is enabled by the Holy Spirit within us so that we have the power to stand against the lesser way. Our spirit is being built up by God while our flesh is being torn down by time, misuse, poor choices, apathy and depression.
Watch and pray to build up your spirit and to bring your body into subjection so that you can live a life commensurate with following after Christ. When you watch and pray it is far less likely that you will be carried away with every wind of doctrine and follow silly philosophies. When you watch and pray it is far less likely that you will fall into temptation. When you watch and pray it is far less likely that you will grab the dangling carrot.
This is a warning to those who are strong in the faith as well as to those who are not. Your spirit is willing to follow after God in every decision, but sometimes your body will convince your mind (rationalize) that what the body wants will somehow fit into what is good and best for your soul. Your body is not a good indicator of right from wrong. It is far better to rely on the written Word of God and get understanding in your spirit by the Holy Spirit than to rely on what your body is telling you.
Think about it. Your body has a life span of maybe 100 years or a little more. Your spirit lives for all eternity. Your body will eventually die. Your spirit will never die. The war between the body and the spirit is real and demands that you use your mind to discern between the two. If your body is winning all the arguments, you will have much troubles in life.