The three most important questions, per Immanuel Kant, are as follows(1)(2)(3):
Does God exist?
Does the afterlife exist?
Do we have free will?
The optimistic answers to these are yes, yes, and yes. The pessimistic answers are no, no, and no.
While I wish for optimism to succeed, sadly I find pessimism to be winning the debate. Here are some quick arguments on behalf of pessimism:
God does not exist
1) If God exists, then no one experiences unbearable suffering.
2) Some people do experience unbearable suffering.
3) Therefore, God does not exist. (modus tollens)
I take premise 2 to be obviously true. The phenomena of suicide proves that suffering becomes unbearable for some people.
But you don’t even need suicide to show this. The suffering experienced at death could very well be unbearable, but because the person is preoccupied with dying they lack the freedom to do anything about it.
Turning to drugs, alcohol, escapist entertainment, etc., is an indication of unbearable suffering, at least when escape from suffering is the motivation (and not, say, fitting in with a certain crowd). “Deaths of despair” include not only deaths from suicide, but deaths from drugs and alcohol too. These inclusions are appropriate because when suffering is unbearable, the thought of suicide can be even worse, which is when folks turn to coping mechanisms. While death is better than unbearable suffering (hence "unbearable"), facing death is itself unbearable. Some people, perhaps most, would rather die than be forced to face death.
Premise 1 is true if a number of assumptions are true.
1) If God exists, then God is a perfect being.
2) A perfect being is perfectly loving and powerful.
3) To be perfectly loving includes desiring all persons to be free from unbearable suffering.
4) There are no overriding reasons to allow unbearable suffering.
Assumption 1 is supported if you think perfect being theism is our best model for God. I think this is plausible; the best arguments for God I've seen are from Josh Rasmussen, who uses perfect being theism. God is usually characterized as worthy of worship, which arguably requires perfection. Certainly, on a Christian conception, God is aware of our suffering, has the power to prevent it, and loves us.
Assumption 2 follows if love and power are good things to have; then of course a maximal being will have these to a maximal degree.
We can motivate assumption 3 by investigating what it means to be loving. To be loving includes being empathetic and compassionate. If someone is maximally empathetic and compassionate, then they will be maximally unable to stand the unbearable suffering of others, and will thus be maximally motivated to prevent or cure that suffering.
Assumption 4 is motivated by placing emphasis on "unbearable". It can make sense for God to desire some suffering on our part, enough to capture theodical goods like soul making. But once suffering becomes unbearable, theodical goods either no longer apply at all, or are outweighed by the suffering. A loving parent might desire for their children to suffer a little to, say, learn the lessons of life, but a loving parent would not want their children to despair of life itself, which is what unbearable suffering entails. The loving parent analogy is especially appropriate for Christianity, where God insists on being called “Father”.
The afterlife does not exist
1) If something is too good to be true, it probably is.
2) A good afterlife is too good to be true.
3) So, there probably is no good afterlife.
4) If there is an afterlife at all, there is a good one.
5) So, there probably is no afterlife.
Premise 4 can be supported by appealing to two intuitions: a) there can only be an afterlife if there is a God to create it; b) God, being perfectly merciful and loving, would create a good afterlife for everyone.
This good afterlife needn't be good for everyone all the time. There could be a sanctification process that requires a great deal of suffering. But the light at the end of the tunnel would prevent this suffering from becoming unbearable; on the other side of sanctification there is paradise, so even this fate is ultimately good.
We can give an abductive version of this where we compare theories to see which one is better. The "no afterlife" theory perfectly predicts the data that no one has yet (unambiguously) come back to life, and the "afterlife" theory can be constructed such that it equally predicts this data. But the explanation given by "No afterlife" is very simple: no one has (unambiguously) come back to life because there is no afterlife; no one survives death. Whatever explanation "afterlife" gives for why no one has (unambiguously) come back to life will be more complex. This complexity will introduce opportunities for this theory to be disproven, making it less likely to be true. Or, on another interpretation of complexity, this complexity increases the mystery of the theory, lowering its explanatory power. Either way, the "no afterlife" theory is the better theory.
There is no free will
1) If God doesn’t have free will, then neither do we.
2) God doesn’t have free will.
3) Neither do we. (modus ponens)
This is a bit of a cheeky argument. The idea is that God can't have free will because God exists necessarily. God has no choice but to exist, and God has no freedom to change his nature. So, God is forced to be what he is and thus God is forced to do what he does. But if anyone has free will, surely a maximally powerful being would. But God doesn't. So, free will must be impossible. We can put it this way:
1) God has no control over his nature.
2) God's nature entails God's actions.
3) By transitivity, God has no control over his actions. (If x entails y, and you have no control over x, then you have no control over y.)
4) If God has no control over his actions, then he isn't free.
5) Therefore, God isn't free.
6) If God, a maximally powerful being, isn't free, then no one is.
7) Therefore, no one is free.
The weakest premise looks like 2. God's nature can entail a range of actions God could take, and God could be free to choose an option from that range. The obvious response to this is that God's choosing is determined by God's nature such that he will always make the choice that he does.
(1) Cf. Antony Flew, There is a God (2007), pg 64
(2) The closest quote of Kant’s to this effect I can find is: “Metaphysics has as the proper object of its enquiries three ideas only: God, freedom, and immortality.” (The Critique of Pure Reason, B 395; https://en.wikiquote.org/wiki/Immanuel_Kant).
(3) Despite the quote in (2), perhaps it's better to categorize these not as important questions on Kant's view but as important postulates. Kant saw God, freedom, and immortality as necessary postulates for morality, and saw morality as the basis for justifying our belief in them. See: https://plato.stanford.edu/entries/kant/.