There are two sides to sanctification in the Christian life. One is mortification, the putting to death of the sinful nature. The other is vivification, the coming to life of the regenerate nature. At the same time that we are putting our flesh to death, we are being revived by the Holy Spirit. These two aspects of sanctification—mortification and vivification—go together. As Calvin put it, “The death of the flesh is the life of the Spirit.”

Christians are to walk with the Spirit: “If we live by the Spirit, let us also walk by the Spirit” (Gal. 5:25). In this verse, as he so often does, the apostle Paul follows an indicative with an imperative; he tells us to become what we are. It is a fact: Those who belong to Jesus live in the Spirit. At regeneration, the Holy Spirit enters the heart of every Christian. Yet we must keep on living in the Spirit, which is precisely what the Galatians were failing to do. Paul had already asked them, “Having begun by the Spirit, are you now being perfected by the flesh?” (Gal. 3:3).

By starting and then stopping in this way, the Galatians had fallen out of step with God’s Spirit. When the apostle speaks of “keeping in step,” he is talking about following orders. The Greek term for “keeping in step” (stoichōmen) comes from the military. It means to stay in formation. First, soldiers would line up in ranks and files. Then, in order to maintain good military discipline, they would stay in line as they marched.

Soldiers not only march in formation, but also run in formation. When they do, there is only one thing they have to worry about, which is keeping in step. They do not need to worry about where they are going, or how they will get there. They do not need to guess how much farther they have to go. Their commanding officer will give them their orders as necessary. The only thing soldiers need to know how to do is step in time. It is the same way in the Christian life. The Holy Spirit is God’s drill sergeant. It is his job to keep us in line. As he barks out the cadence, all we have to do is keep our place in the formation, running in step with his commands.

This analogy shows us where we ought to be in relation to other Christians. We do not run alone. Our brothers and sisters are right beside us. Ideally, we are matching them stride for stride. As long as we maintain good discipline, there will not be any pushing and shoving in the ranks, the kind of “provoking” and “envying” that Paul warns about in Galatians 5:26. Instead, by staying in formation, we will maintain our unity in the Spirit. A good unit never lets one of its men fall behind. If a soldier stops running because of injury, discouragement, or fatigue, his buddies will circle around and gather him back into his unit. So also in the church we are called to maintain unity by going back to help those who have fallen.

Keeping in step takes discipline, and so does spiritual growth. The Holy Spirit rarely works in extraordinary ways. Instead, he uses the ordinary means of grace to bring spiritual growth: the reading and preaching of God’s Word, the sacraments of baptism and communion, and the life of prayer. True spiritual growth comes from walking with the Spirit every day until, finally, keeping in step with him becomes a holy habit.

J. I. Packer’s explanation of how the Spirit works is worth quoting at length:

The Spirit works through means—through the objective means of grace, namely, biblical truth, prayer, fellowship, worship, and the Lord’s Supper, and with them through the subjective means of grace whereby we open ourselves to change, namely, thinking, listening, questioning oneself, examining oneself, admonishing oneself, sharing what is in one’s heart with others, and weighing any response they make. The Spirit shows his power in us, not by constantly interrupting our use of these means with visions, impressions, or prophecies … (such communications come only rarely, and to some believers not at all), but rather by making these regular means effective to change us for the better and for the wiser as we go along.… Habit forming is the Spirit’s ordinary way of leading us on in holiness.… Love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control are all of them habitual … ways of thinking, feeling, and behaving.

Packer goes on to stress that “Holiness by habit forming is not self-sanctification by self-effort, but is simply a matter of understanding the Spirit’s method and then keeping in step with him.” This is how God grows good spiritual fruit. The more we keep in step with the Holy Spirit through the Word, sacraments, and prayer, the more fruitful we become.