“Star Trek was an attempt to say that humanity will reach maturity and wisdom on the day that it begins not just to tolerate, but take a special delight in differences in ideas and differences in life forms. […] If we cannot learn to actually enjoy those small differences, to take a positive delight in those small differences between our own kind, here on this planet, then we do not deserve to go out into space and meet the diversity that is almost certainly out there.” ― Gene Roddenberry
It's a series filled with hope for the future, that never says getting to that place will be easy. It very consistently tackles social issues such as religion, gender identity, capitalism, and the relations of power. For example the Prime Directive - that Starfleet shall not interfere in the independent development of other cultures - is a classic ML vs anarchist problem. Does a powerful socialist state have the right to invade foreign planets and force them to conform to their ideas? The show runs into other species that challenge both sides of this debate with great frequency. For example the Borg are a communist hivemind that force others to join their hivemind through violence in their pursuit of distinctiveness to add to their swarm. A dark mirror to the Federation that shows what things would be like without the prime directive. Or, the Bajorans, a deeply religious species that were enslaved by the Cardassians for 50 years, who are weighing their options of whether or not they want to join the Federation now they're free, and some are questioning why the Federation hasn't done more for them.