The Catechism defines heaven as the “ultimate end and fulfillment of the deepest human longings, the state of supreme definitive happiness” (1024).
The textbook answer is the knowledge that we will have of the divine essence, which theologians call the beatific vision. St. John writes about it in 1 John 3:2: “Beloved, we are God’s children now; it does not yet appear what we shall be, but we know that when he appears we shall be like him, for we shall see him as he is.”
In his 1336 Apostolic Constitution Benedictus Deus, Pope Benedict XII defined this vision as
[Seeing] the divine essence by intuitive vision, and even face to face, with no mediating creature, serving in the capacity of an object seen, but divine essence immediately revealing itself plainly, clearly, and openly, to them.
By intuitive Benedict XII means this vision is a knowledge that is higher than all abstraction, discursive reasoning, and analogy. By immediate he means that we will know God’s essence without any mental image or created idea that merely represents the divine essence.
Just as the form of a dog is immediately united to my intellect when I know a dog, so too the divine essence will be immediately united to my intellect when I know God’s essence in the beatific vision. Rather than knowing a similitude of the divine essence, I will know the divine essence itself.
This immediate knowledge of God’s essence is what constitutes man’s perfect happiness—hence the name beatific (Latin for happy). The reason is because the intellect attains its complete perfection. And it does so in two ways.
First, it comes to know the essence of its ultimate end, that which it was created to know. Second, it arrives at the terminus of all intellectual inquiry. Because God is that than which nothing greater can be known, knowledge of his essence leaves the intellect with no further desire to acquire knowledge for its perfection.
Consider how when we seek to understand something we either look to the thing itself for answers to our questions or to something outside it. Take a tree, for example. We may ask, “What makes its leaves green?” The answer is chlorophyll. We may then ask, “Why do the leaves have chlorophyll?, and answer because the tree’s genes tell the tree to make chlorophyll. But why do its genes tell it to make chlorophyll? The answer is because the leaves need to make energy for the tree, and they use chlorophyll to do that.
Notice that to answer these questions we didn’t have to appeal to anything outside the tree.
But what if we ask, “How do the leaves make energy?” Unlike the other questions, we must appeal to something outside the tree to answer this one: Leaves make energy using light from the sun. They do this using chlorophyll in the process called photosynthesis.
Even the tree’s very existence must be explained by something outside itself. We know the tree doesn’t exist by nature—if it did, there would never be a time when the tree didn’t exist! So we must appeal to something else.
What all this means is that any reality that depends upon something else for its intelligibility leaves our intellect unsatisfied. The only thing that can fully satisfy its quest for truth is something that doesn’t rely on anything outside itself in order to be known. Knowing the essence of such a reality would leave the intellect desiring nothing else, thus perfecting it and constituting complete human happiness.
And this reality is God.
It’s important to note that the beatific vision—the intuitive and immediate knowledge of God’s essence—is not comprehensive. Our knowledge can’t exhaust the divine essence. Only God can fully know himself, as he does in the persons of the Trinity. It requires infinite intellective power to know infinite being.
So how do the saints know God perfectly but not fully? Consider how two people may know the same truth, but know it more or less profoundly.
For example, someone may know that God exists based on reasonable belief. He looks out into the world and sees a great complexity and order that extends all the way back to the beginning of the universe. And since complexity and order are ordinarily explained by intelligence, this person concludes that a super intelligence, like God, is responsible for making the universe. This is a reasonable belief.
Another person, however, might know the same truth—that God exists—but know it by way of metaphysical demonstration. He says, “I know God exists because it’s a matter of metaphysical necessity that he exists. For without him, nothing would exist.
In these two examples, we see that the same object can be known in accord with the mode of the knower. Both God and the saints know the divine essence, but in essentially different ways: according to the mode of the knower.
God’s intellective power is infinite, so he knows the divine essence in an infinite way. The blessed, however, know the divine essence in a way that is consistent with a finite intellect: they know it in a limited way. Although they have a real knowledge of God’s essence, their knowledge doesn’t exhaust it.
The knowledge that we can have of God on this side of the veil is real knowledge, and can be a source of intellectual delight. But it pales in comparison to the delight that we will have when the intellect finally rests in seeing God face to face in the beatific vision and our rational natures are ultimately fulfilled.