This is such a loaded question.
Many philosophers tried to answer it, some reaching an optimistic vision, while others concluding it was all doom and gloom (nihilists etc.). There are so many answers, because there are so many variables. To touch the point of the iceberg it depends for example if you believe in the existence of a prime cause (aka God and thus some kind of spiritual dimension) or not; and if yes, how you imagine its attitude (is it sentient? If yes, is it neutral, good or evil? How much does it interfere in the life of the universe? And so on and on...). That shapes how you start to analyze the purpose of human life and existence as a whole. So let's stick on the dilemma of being atheist or theist (avoiding the gray areas like being agnostic, because I'm trying to keep it super simple):
If you say no, there is no God, I believe in the chaotic order that is evolution: you may think for example that the purpose of life is hedonism, the pursuit of pleasure, because you live only once, in the now. And then you enter the rabbit hole of "what kind of hedonism?".
If you say yes and God is caring, the purpose of life is usually the betterment of your soul, to trascend the material dimension. There is an internal evolution here at play. And here the question then is how? You will have to pick which interpretation of the spiritual world convinces you more, a traditional religion or an esoteric philosophy (theosophy etc.).
If your God is neutral, then you probably have full agency, you choose what is you life purpose. You are free to make you own meaning, to play in this playground. The choices here become infinite like the stars.
If your God is evil, like the Gnostic Demiurge, your goal will be to rebel against the current system of oppression, escaping the prison it built to keep you in, for example reaching the true God or becoming your own God.
And, I stress it once more, this is only one of many variables.
In the end nobody knows. This is a personal quest with answers based on your personal intimate experience on what is life, what you currently value, what do you think is it worth living for and the type of relationship you have (or have not) built with its incorporeal, abstract, intuitive, invisible side. Fools are the ones that claim their answer is the only right one, because humanity in its current state does not possess the tools to prove or confute anything on this subject with undeniable evidence. If not fools, then they are manipulative bastards, that want to take advantage of your confusion and lack of direction to persuade you into pursuing a goal that does not benefit you, but them.