If we honestly consider within ourselves how very blind our thought is when faced with the heavenly secrets of God, and how very unfaithful our heart is in everything, we shall not doubt that faith is infinitely beyond all the power of our nature and that it is an extraordinary and precious gift of God. For as Saint Paul says, “Nobody knows the things of man, except for the spirit of man which is in him; and nobody knows the things of God except the Spirit of God” (1 Corinthians 2:11) If the truth of God wavers in us, even in things that our eye sees, how could it be firm and stable in things which the Lord promises our eye cannot see nor our intelligence grasp?
It is clear, then, that faith is an illumination from the Holy Spirit which enlightens our minds and strengthens our hearts; it settles us in this assurance — that the truth of God is so certain that He will accomplish everything that His holy Word has promised He will do.
That is why the Holy Spirit is described as being a guarantee which confirms in our hearts the certainty of divine truth, and as a seal sealing our hearts while we look forward to the day of the Lord.
21 Now He who establishes us with you in Christ and anointed us is God, 22 who also sealed us and gave us the Spirit in our hearts as a pledge. 2 Corinthians 1:22
13 In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise, 14 who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory. Ephesians 1:13, 14
John Calvin, Truth for All Time