Love is the guard of Christian freedom. The Holy Spirit is its guide. These principles accomplish what the law could never do. It withheld liberty, and yet did not give purity. The Spirit of love and of sonship bestows both, establishing a happy, ordered freedom, the liberty of the sons of God.
From the first of these two factors of Christian ethics the Apostle passes in ver. 16 to the second. He conducts us from the consequence to the cause, from the human aspect of spiritual freedom to the Divine. Love, he has said, fulfils all laws in one. It casts out evil from the heart; it stays the injurious hand and tongue; and makes it impossible for liberty to give the rein to any wanton or selfish impulse. But the law of love is no natural, automatic impulse. It is a Divine inspiration. "Love is of God." It is the characteristic "fruit of the Spirit" of adoption (ver. 22), implanted and nourished from above. When I bid you "by love serve each other," the Apostle says, I do not expect you to keep this law of yourselves, by force of native goodness: I know how contrary it is to your Galatic nature; "but I say, walk in the Spirit," and this will be an easy yoke; to "fulfil the desire of the flesh" will then be for you a thing impossible.
The word _Spirit_ (=pneumati=) is written indefinitely; but the Galatians knew well what Spirit the Apostle meant. It is "the Spirit" of whom he has spoken so often in this letter, the Holy Spirit of God, who had entered their hearts when they first believed in Christ and taught them to call God Father. He gave them their freedom: He will teach them how to use it. The absence of the definite article in _Pneuma_ does not destroy its personal force, but allows it at the same time a broad, qualitative import, corresponding to that of the opposed "desire of the flesh." The walk governed "by the Spirit" is a _spiritual_ walk. As for the interpretation of the _dative_ case (rendered variously _by_, or _in_, or even _for the Spirit_), that is determined by the meaning of the noun itself. "The Spirit" is not the path "in" which one walks; rather He supplies _the motive principle, the directing influence_ of the new life.[139] Ver. 16 is interpreted by vv. 18 and 25. To "walk in the Spirit" is to be "led by the Spirit"; it is so to "live in the Spirit" that one habitually "moves" (_marches_: ver. 25) under His direction.