The wind is heard among the branches of the trees, rustling the leaves and flowers; yet it is invisible, and no man knows whence it comes or whither it goes. So with the work of the Holy Spirit upon the heart. [The Spirit] can no more be explained than can the movements of the wind. A person may not be able to tell the exact time or place, or to trace all the circumstances in the process of conversion; but this does not prove him to be unconverted.
By an agency as unseen as the wind, Christ is constantly working upon the heart. Little by little, perhaps unconsciously to the receiver, impressions are made that tend to draw the soul to Christ. These may be received through meditating upon Him, through reading the Scriptures, or through hearing the Word from the living preacher. Suddenly, as the Spirit comes with more direct appeal, the soul gladly surrenders itself to Jesus. By many this is called sudden conversion; but it is the result of long wooing by the Spirit of God--a patient, protracted process. While the wind is itself invisible, it produces effects that are seen and felt. So the work of the Spirit upon the soul will reveal itself in every act of him who has felt its saving power. When the Spirit of God takes possession of the heart, He transforms the life. Sinful thoughts are put away, evil deeds are renounced; love, humility, and peace take the place of anger, envy, and strife. Joy takes the place of sadness, and the countenance reflects the light of heaven. No one sees the hand that lifts the burden, or beholds the light descend from the courts above. The blessing comes when by faith the soul surrenders itself to God. Then that power which no human eye can see creates a new being in the image of God.
It is impossible for finite minds to comprehend the work of redemption. Its mystery exceeds human knowledge; yet he who passes from death to life realizes that it is a divine reality. The beginning of redemption we may know here through a personal experience. Its results reach through the eternal ages.--The Desire of Ages, pp. 172, 173. YRP 15
There is no limit to the usefulness of one who, by putting self aside, makes room for the working of the Holy Spirit upon his heart, and lives a life wholly consecrated to God. DA 250-251
The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit. John 3:8 (King James Version)