[Abyss, in classic Greek, is equivalent to 'bottomless,' 'unbounded,' 'the pit,' 'the immeasurable depth.' Hence, (1) of `the deep' sea: Gen. 1:2; 7:11; Deut. 8:7; (2) of Orcus (a very deep gulf or chasm in the lowest parts of the earth: Ps. 71:20); both as the common receptacle of the dead, Rom. 10:7, and especially as the abode of demons, Luke 8:31; Rev. 9:1f; Rev. 9:11; 11:7; 17:8; 20:1,3].
Calvin used the word abyss in a very interesting and unique way. Though the word 'abyss' often appears in Scripture as an adjective that describes the vastness of the ocean depths, or with reference to the description for the location of hell (Romans 10:7), especially in the book of Revelation, Calvin used it almost entirely to refer to that incomprehensible void that exists between the person of God and the human mind's inability to understand the person of God.
[Calvin’s] Institutes of the Christian Religion begins with this affirmation:
“The entire sum of our wisdom, of that which deserves to be called true and certain wisdom, may be said to consist of two parts: namely, the knowledge of God, and of ourselves ... Reformation theology insists upon an understanding that the God of Creation is a God of complete otherness in relation to man. Formulating thoughts about His nature and character can only be uninformed if the source of that truth about Him does not arise from His own revelation of Himself. The only way any human being can rightly understand God is if God reveals Himself to us. He is completely beyond our comprehension. All that can ever come from the mind of men regarding the character of God [apart from God's revelation of Himself] is confusion, misrepresentation, and foolishness ...”
Further on In the Christian Institutes he added
But if they do not admit that whatever happens in the universe is governed by God's incomprehensible plans, let them answer to what end the Scripture says that His judgments are a deep abyss [Ps.36.6]. For since Moses proclaims that the will of God is to be sought not far off in the clouds or in abysses, because it has been set forth familiarly in the law [Deut. 30:11-14], it follows that H has another hidden will which may be compared to a deep abyss; ... but since God illumines the minds of his own with the spirit of discernment [Job 20:3 or Isa. 11:2] for the understanding of these mysteries which he has designed to reveal His Word, now no abyss is here: rather a way in which we ought to walk in safety, and a lamp to guide our feet [Ps. 118:105, Vg. 119: 105, EV], the light of life [cf. John1:4; 8:12], and the school of sure and clear truth. Yet his wonderful method of governing the universe rightly is called an abyss, because while it is hidden from us, we ought reverently to adore it. Institutes, 212-213.
The uniqueness of Christianity is that among the religions of the world, it teaches that because God is completely other and incomprehensible to the human mind, no one may know God without the revelation of Himself in the person of Jesus Christ. This is why Christ said to His disciples, ‘He who has seen Me has seen the Father (John 14:9).’ Statements like this have led other religions of the world to wrongly assess Christianity to be a religion steeped in intolerance and narrowness. But it is not that Christ is being intolerant to other ideas about God as much as He is saying that apart from Himself, it is impossible to comprehend God because ‘I am the perfect revelation of God.’
Calvin wrote that ‘[God’s] essence is so incomprehensible that His majesty is hidden, remote from all our senses. (Calvin: Origin and Development Of His Religious Thought; Wendel, Baker Books, 1997, 151-152).
In his commentary on the First Epistle of Peter, Calvin wrote:
For, since God is incomprehensible, faith could never reach to him, except it had an immediate regard to Christ. Nay, there are two reasons why faith could not be in God, except Christ intervened as a Mediator: first, the greatness of the divine glory must be taken to the account, and doubtless very far from being capable of ascending so high as to comprehend God. Hence all knowledge of God without Christ is a vast abyss which immediately swallows up all our thoughts.
A clear proof of this we have, not only in the Turks and the Jews, who in the place of God worship their own dreams, but also in the Papists. Common is that axiom of the schools, that God is the object of faith. Thus of hidden majesty, Christ being overlooked, they largely and refinedly speculate; but with what success? They entangle themselves in astounding dotages, so that there is no end to their wanderings. For faith, as they think, is nothing else but an imaginative speculation.
Let us, therefore, remember, that Christ is not in vain called the image of the invisible God, (Colossians 1:15;) but this name is given to him for this reason, because God cannot be known except in him. [...] It is hence evident that we cannot believe in God except through Christ, in whom God in a manner makes himself little that he might accommodate himself to our comprehension; and it is Christ alone who can tranquillize consciences, so that we may dare to come in confidence to God. [verse @ 1 Peter 1:21].
In fact, so restricted and incapable are we from even beginning to comprehend the majesty of the person of God, that all attempts to describe Him apart from Christ find us in an abyss of ignorance...We must also notice what he says, that we have been called out of darkness into God’s marvelous or wonderful light; for by these words he amplifies the greatness of divine grace. If the Lord had given us light while we were seeking it, it would have been a favor; but it was a much greater favor, to draw us out of the labyrinth of ignorance and the abyss of darkness. We ought hence to learn what is man’s condition, before he is translated into the kingdom of God.
And this is what Isaiah says, 'Darkness shall cover the earth, and gross darkness the people; but over thee shall the Lord be seen, and his glory shall in thee shine forth' (Isaiah 60:2)... And truly we cannot be otherwise than sunk in darkness, after having departed from God, our only light. [verse @ 1 Peter 2:9]
This revelation that comes upon the human understanding that God is found only in the person of Christ, happens only because of God’s gracious bestowing of that revelation of Christ to us. If man truly lies within an abyss of ignorance, then only grace can remove him from it.
... that no knowledge of Christ can be obtained, until the Father enlighten by his Spirit those who are by nature blind; and yet that it is in vain to seek God, unless Christ go before; for the majesty of God is so lofty, that the senses of men cannot reach him. Nay, more, all that knowledge of God which men may think that they have attained out of Christ will be a deadly abyss. When he says that he alone hath known the Father, he means that it is an office which belongs peculiarly to himself, to manifest God to men, who would otherwise have been concealed (229, John).
It hence follows, that the grace of God penetrates into the abyss of death, if only it be sought there; so that it is not by any means to be withheld from the Gentiles (307, Romans).
... for there is no one who anticipates the Lord; but we are all, without exception, delivered by his free mercy from the deepest abyss of death, when there is no knowledge of him, no desire of serving him, in a word, no conviction of his truth (315, Romans).
What an amazing statement this last comment is! Calvin says that there is no one ‘expecting or awaiting’ the Lord. Everyone lives in this abyss of ignorance about God and the whole course of providence. There are no exceptions, except that to be delivered from the abyss of ignorance is to know Christ! His mercy rescues us from the vast abyss of ignorance, even when there is no knowledge of Him in us! Even when we have nothing to offer as meriting this mercy! Even when we do not know the truth about Him, but are rather lost in this abyss of spiritual darkness, He rescues us. We are more than simply lost though according to Calvin. We are in cahoots with another god … The god of this world.
O that we could consider this, according to what it doth import and carry in it of horror and detestableness! It is a thing that we do not yet believe, that a world inhabited by reasonable creatures, God’s own offspring, is universally fallen into a confederacy and combination with another god, with an enemy god, an adversary god, against the living and true God! Men have changed their God. And what a fearful choice have they made! Fallen into a league with those wicked creatures that were weary of his government before, and that were, thereupon, thrown down into an abyss of darkness, and bound up in the chains thereof, unto the judgment of the great day. But doth the Scripture say this in vain? or hath it not a meaning when it calls the devil the god of this world? O with what amazement should it strike our hearts, to think that so it is, that the whole order of creatures is gone off from God, and fallen into a confederacy with the devil and his angels, against their rightful sovereign Lord. [footnote @ 2 Corinthians 4:4]