Sanctification is the only certain evidence of that indwelling of the Holy Spirit which is essential to salvation.
“If any man has not the Spirit of Christ — he is none of His” (Romans 8:9).
The Spirit never lies dormant and idle within the soul. He always makes His presence known by the fruit He causes to be borne in heart, character and life. “The fruit of the Spirit,” says Paul, “is love, joy, peace, patience, gentleness, goodness, faith, meekness, temperance” and such like (Galatians 5:22). Where these things are to be found — there is the Spirit; where these things are lacking — men are dead before God.
The Spirit is compared to the wind; and, like the wind, He cannot be seen by our bodily eyes. But, just as we know there is a wind by the effect it produces on waves and trees and smoke — so we may know the Spirit is in a man — by the effects He produces in the man’s conduct. It is nonsense to suppose that we have the Spirit — if we do not also “walk in the Spirit” (Galatians 5:25). We may depend on it as a positive certainty, that where there is no holy living — there is no Holy Spirit! The seal that the Spirit stamps on Christ’s people is sanctification. As many as are actually “led by the Spirit of God, they,” and they only, “are the sons of God” (Romans 8:14).
by J.C. Ryle