John Calvin Commentary John 3:8

"The wind bloweth where it will, and thou hearest the voice thereof, but knowest not whence it cometh, and whither it goeth: so is every one that is born of the Spirit." — John 3:8 (ASV)

The wind blows where it pleases. This is not to say, strictly speaking, that there is will in the blowing, but because its movement is free, uncertain, and variable; for the air is carried sometimes in one direction and sometimes in another. This is how it applies to the matter at hand; for if it flowed in a uniform motion like water, it would be less miraculous.

So is every one that is born of the Spirit. Christ means that the movement and operation of the Spirit of God in human renewal is no less perceptible than the motion of the air in this earthly and outward life, but that its manner is concealed; and that, therefore, we are ungrateful and malicious if we do not adore the inconceivable power of God in the heavenly life, which is so strikingly displayed in this world, and if we attribute less to Him in restoring the salvation of our soul than in upholding the bodily frame. The application will be somewhat more evident if you rephrase the sentence this way: Such is the power and efficacy of the Holy Spirit in the renewed person.