Jean Calvin, answered from the texts and cited to the page.
The Lord himself gave us the figure: the word of God is seed.1 As seed cast upon the sea-shore dies without fruit, but cast upon cultivated ground brings forth an abundant increase, so the word, falling upon a soul prepared by the heavenly Spirit, is abundantly fruitful. Paul draws the same lesson from the Corinthians — he who plants and he who waters are nothing; it is God who gives the increase.2
But the figure must be pressed further, or it misleads. The seed does not germinate by its own warmth. Our minds are blind by nature, and the word, however plainly preached, cannot penetrate them unless the internal teacher, the Spirit, makes way for it by his illumination.3 The schoolmen erred precisely here: they treated faith as a simple assent of the understanding, as though hearing were sufficient.
It is not.4 Faith is a singular gift of God in two respects — the mind is enlightened to understand the truth, and the heart is established and confirmed in it; the Holy Spirit not only originates faith but increases it by degrees, until he conducts us all the way to the heavenly kingdom.5 So when you ask what the seed of faith is, the answer has two parts that must be held together.
The Spirit of truth retains and fixes our faith on Christ alone, delivering the mind from doubt and the fear of being deceived; and the apostles, as organs of that same Spirit, bore the word outward so that faith comes by hearing — yet derives its certainty from the seal and earnest of the Spirit.67 Those who imagine faith is formed naturally by preaching alone do not sufficiently know the darkness of the human mind.
Those who disdain outward preaching and seek only secret revelations are fanatics who have cut the root from the plant. Christ joins these two things together, and we must not separate what he has joined.8 The seed, then, is the word; the soil is the heart prepared by grace; the germination is the Spirit's own work. None of the three may be subtracted.
as seed, if it fall on a desert and neglected spot of ground, will die without producing any crop, but if it be cast upon a well manured and cultivated field, it brings forth its fruit with an abundant increase,—so the word of God, if it fall upon some stiff neck, will be as unproductive as seed dropped upon the sea-shoreInstitutes of the Christian Religion Vol 2
Neither is he that planteth any thing, neither he that watereth; but God, that giveth the increase.Institutes of the Christian Religion Vol 2
The word of God is like the sun shining on all to whom it is preached; but without any benefit to the blind. But in this respect we are all blind by nature; therefore it cannot penetrate into our minds, unless the internal teacher, the Spirit, make way for it by his illumination.Institutes of the Christian Religion Vol 1
the schoolmen are altogether erroneous, who, in the discussion of faith, regard it as a simple assent of the understanding, entirely neglecting the confidence and assurance of the heartInstitutes of the Christian Religion Vol 1
Faith, therefore, is a singular gift of God in two respects; both as the mind is enlightened to understand the truth of God, and as the heart is established in it. For the Holy Spirit not only originates faith, but increases it by degrees, till he conducts us by it all the way to the heavenly kingdom.Institutes of the Christian Religion Vol 1
The Spirit is said to testify of Christ, because he retains and fixes our faith on him alone, that we may not seek elsewhere any part of our salvation... wherever he speaks, he delivers the minds of men from all doubt and fear of being deceived.Commentary on the Gospel according to John Vol 2
we now see in what way faith is by hearing, (Rom. x. 17,) and yet it derives its certainty from the seal and earnest of the SpiritCommentary on the Gospel according to John Vol 2
we see how Christ joins these two things together; and, therefore, though there is no faith till the Spirit of God seal our minds and hearts, still we must not go to seek visions or oracles in the cloudsCommentary on the Gospel according to John Vol 2