“Walking is a metaphor used from time to time in Scripture to denote spiritual progress. People in the first century could not travel as fast as we do, with our cars, planes, trains and the like, but even so, for them as for us, walking was the slowest way of going places. But even though walking was slow and unspectacular, walking meant progress. If anyone kept walking, she or he would certainly cover the ground and eventually reach the destination. So, for the apostle walking was an apt metaphor. If any believer was walking, that believer was going somewhere.” In this case, walking in the Spirit (Galatians 5:16) meant walking in the life of Christ, the will of God. We are going somewhere in our relationship with him.
According to 1Corinthians 2:11-16, believers have two spirits. Like all mankind, we have the spirit of man (or the world), and by it we can understand the things important to men or this world. However, believers are also given the Spirit of God, which equips us to understand the things of God. All men do not have the Spirit of God; only believers have God’s Spirit. To walk in the Spirit (Galatians 5:16) concerns walking or living (communicating or acting) according to the prompts of this Spirit, as opposed to the spirit of man which all men received at birth. We received God’s Spirit, according to Paul, after we placed our trust in Christ (Galatians 3:2; 4:6). Because the Spirit of God dwells in us, we are able to walk **in** the Spirit or according to the desires of the Spirit of God. However, this does require our being yielded to the Spirit otherwise our walk would be in the flesh—or according to the spirit man received at birth. If we are yielded to God’s Spirit within us, our pattern of life will be different from how it would be if we walked according to the flesh. Walking after the Spirit would be living according to the original pattern God intended (cf. Genesis 1:26-27).
God promised those who walk by the Spirit that we won’t fulfill the lusts of the flesh (Galatians 5:16)! The Spirit within us influences our lives in a number of ways. It could be done through the Bible or God’s word (John 14:26). We could be influenced by others who follow Christ (1Corinthians 11:1) or simply through an inner impulse or sense of direction (Matthew 10:19-20; 1John 2:20, 27). But, one may ask: “How can I tell that I’m walking in the Spirit?” We were told by Jesus that the Spirit speaks to us of Jesus (John 14:16-17, 26; 15:26; 16:13-15). Therefore, if we are walking in the Spirit, we shall be speaking similar to the manner in which Christ spoke, and we will be living similar to how Christ lived. When others see us, they will see a disciple or follower of Christ—an image of Christ or God (cf. Genesis 1:26-27).
Walking in the Spirit is neither by law nor is it by license (unchecked behavior). It is by faith operating through love (Galatians 5:6). I like the way I heard it explained by another man. “It’s as if we are a computer, and we have two hard drives in us. One is programmed according to the Spirit, and the other is programmed according to the flesh. In any given situation, it’s up to us to decide which ‘drive’ we will access. The resources of the Spirit (operate one drive), while the resources of the flesh (operate the other drive). Our work concerns the drive we access. Should we operate the ‘drive’ of our flesh and make life as efficient as possible? God never meant our system to run that way. He wants us to run off the ‘drive’ of the Spirit of God. We could say that the law is like an error message that keeps popping up on our flesh ‘drive.’ It doesn’t fix the drive, and it sometimes makes the system crash – but it does tell us something is wrong, and it points us in the right direction. However, the Spirit ‘drive’ has programming on it that will make our lives operate better in God’s sight, and one day God will replace the ‘flesh’ drive with a resurrection upgrade.”
 Leon Morris, Galatians: Paul’s Charter of Christian Freedom; page 167; Downers Grove, Ill.; InterVarsity Press, 1996.