Walking in the Spirit.
But more particularly, this walking in the Spirit consists in the habitual exercise of faith in Christ; that faith by which we are united to him, so as to receive out of his fullness even grace for grace. Christ is made of God unto us sanctification, as well as redemption; and it is by faith in him that our sanctification is advanced; for, says the apostle,
I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. (Galatians 2:20)
And this corresponds with His own language to the disciples,
Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me. I am the vine, ye are the branches. He that abideth in me, and I in him, the same bringeth forth much fruit: for without [or, out of] me ye can do nothing. If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.” (John 15:4-6)
Now, “we abide in Christ” when “his word abides in us.” It is by faith that we are first united to Christ, and it is by the continued exercise of the same faith that our union with him is maintained, and that we derive from him, as a branch draws sap from the vine, the nourishment which makes us fruitful. It was “the truth as it is in Jesus” that was the means of our conversion, and it is the same truth that is the instrument of our progressive sanctification; for Christ’s prayer for his disciples, even when he spoke of the promise of the Spirit, was, “Sanctify them through thy truth: thy word is truth.” (John 17:17) And the truth here spoken of is not solely, nor even chiefly, the truth contained in the law, although that is useful, as affording a perfect rule and authoritative directory for the conduct of life; but it is especially the truth contained in the Gospel; for that affords the most constraining motives to a life of new obedience; and “what the law cannot do, seeing that it is weak through the flesh,” the Gospel can accomplish, because it is, in the hand of the Spirit, an effectual means of sanctification.
We are not only justified, we are sanctified also by the truth as it is in Jesus; and they who are jealous of the doctrine of free grace, because of its supposed tendency to relax the obligations of holiness, betray a lamentable ignorance at once of the scheme of revealed truth, and the actual experience of all believers. Man’s method of sanctification is by the law, God’s method of sanctification is by the Gospel; the former is by works, the latter is by faith, unto works.
(The Evangelical Presbyterian, January 2010)
(The Glories Of Christ, Chicago: Moody Press, 1949, p. 55-56)