One of the strands of Christian thought maintains that God's mercy is such that everybody will eventually be saved. This is called universalism and goes back at least as far as the 3rd century theologian Origen. It's a tempting doctrine, especially for those who focus on the love of God more than the truth of God, but it does have certain problems.
Before considering those, let's look at its attractions and how it might work. The main attraction is obviously that everything is working for the good and no one will be left behind. Somehow every human soul will be brought to salvation. However long it takes, everyone gets to heaven. This might work through a long period of purgatory or it could work through reincarnation with souls continuously coming back until they learn their lessons and are purified of their sins. God has all the time he wants to bring this about and so there is no reason why it cannot come to pass.
The chief objection to universalism is that it comes close to making a mockery of free will which is the defining characteristic of a spiritual being and the reason why God created human souls. It's why we live in a world of good and evil where we have to make a positive choice and the fact of God is not self-evident. We have to incline our hearts to him of our own accord.
This objection might be got around by saying that all those who are saved do eventually have to choose God and not reject him, but if you reject God constantly and only finally accept him because circumstances force you to do so, is this really free will? Surely the whole point is that you choose God when you don't have to, when there are other options that might seem preferable? This act of choice, freely made without coercion or experiencing the results of wrong choices, indicates what you are like inside as a person. If it is only endless experiences in purgatory or a sequence of earthly lives that finally bring you round to God when you have more or less exhausted every other possibility, is that really your own choice, your free choice? Does it reflect who and what you are? You see the problem. If all are saved then salvation is devalued. If it happens anyway then why bother to make spiritual efforts? You might get there earlier but so what? Sin away in perfect freedom, thumb your nose at God who is just a benevolent old uncle who tut tuts affectionately but doesn't really mind what you get up to.
There is something unusual about the present time. It does seem, more than in the past, as if human beings are being called to make a definitive choice, though whether this is for all time or the foreseeable future is a moot point. But it could be that there is a parting of the ways with those who do choose to follow God being taken to higher ways of being while those who either actively deny him or just can't make the leap of faith being held back and left on lower levels. Jesus talks of sheep and goats. He didn't seem to be a universalist. He warns us of the perils of hell in language that leaves no room for doubt. On the other hand, God is merciful and the parable of the prodigal son might offer hope to everyone.
Perhaps to think in terms of salvation and damnation is a mistake. Which is not to say those two conditions do not exist. I'm sure they do and we'd better believe it. But maybe there are many worlds between these two extremes where souls that have neither embraced the reality of God nor sunk to total rejection of him experience external environments that correspond to the inner state of their souls. Until such time as they change that inner state though that may be more difficult in worlds other than this one which is specifically set up for the purpose of choice and change.
What this all comes down to is time. How much time are we allowed? Endless time or a certain period? I can think of reasons for either but only God knows. There is surely a sense, though, that it would be wrong to allow time to play no part in the process. A soul that either ignores or rejects God constantly must one day be called to account in a world in which justice has any meaning. God's mercy may precede his wrath but does it completely obviate it?