Question: "What does it mean to walk in the Spirit?"
Believers have the Spirit of Christ, the hope of glory within them (Colossians 1:27). Those who walk in the Spirit will show forth daily, moment-by-moment holiness. This is brought about by consciously choosing by faith to rely on the Holy Spirit to guide in thought, word, and deed (Romans 6:11-14). Failure to rely on the Holy Spirit's guidance will result in a believer not living up to the calling and standing that salvation provides (John 3:3; Ephesians 4:1; Philippians 1:27). We can know that we are walking in the Spirit if our lives are showing forth the fruit of the Spirit which is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control (Galatians 5:22,23). Being filled (walking) with the Spirit is the same as allowing the word of Christ (the Bible) to richly dwell in us (Colossians 3:16).
The result is thankfulness, singing, and joy (Ephesians 5:18-20; Colossians 3:16). Children of God will be led by the Spirit of God (Romans 8:14). When Christians choose not to walk in the Spirit, thereby sinning and grieving Him, provision has been made for restoration through confession of the wrongdoing (Ephesians 4:30; 1 John 1:9). To walk in the Spirit is to follow the Spirits leading. It is essentially to walk with the Spirit, allowing Him to guide your steps and conform your mind. To summarize, just as we have received Christ by faith, by faith He asks us to walk in Him, until we are taken to heaven and will hear from the Master, "Well done!" (Colossians 2:5; Matthew 25:23).