Hello, this is an excellent question and surprisingly has a very clear answer, at least for Greek and western civilization!

In Ancient Greece, the universe was conceived through Aristotelianism. Within this system, all planetary bodies (including the moon) were apart of the celestial realm. Any body inside in the celestial realm was considered perfect, and completely spherical and round. This meant that there would be no perceived imperfections on the moon.

In the Aristotelian universe, everything was viewed through a geometric lens, wherein the universe was perfectly ordered through uniform circular motions. At a more macro level, the universe was divided between the terrestrial and the celestial realm. The terrestrial realm was anything below the sphere of the moon. It followed as such; the earth began with a sphere of terrestrial matter, followed by a sphere of water, a sphere of air, fire and lastly the aether. After the aether, we entered the celestial realm of the planets where everything was supposed to be perfect. ( this was everything above and including the sphere of the moon ).

Later, in the medieval world, these views were made to fit the Christian conception. The earth was the imperfect realm and the heavens were perfect (including the moon). So in regards to your question, Galileo’s discovery of the moon’s craters created an uproar within the western world. Religious officials fought hard to disprove Galileo’s claim by deploying Jesuit astronomers to come up with counter parts to his discovery. The stakes were increasingly high for the Catholic Church because if the celestial realm was not perfect, heaven would not be either. If you would like to research some figures involved in the Catholic Church at this time dealing with these issues an investigation of Pope Urban VIII and Cardinal Bellarmine would be fruitful!

In many ways, the Greek conception of the universe only began to be dismantled starting from the creation of the telescope by Hans Lepershay. Following his creation, Galileo began experimenting with the device, and used it as an instrument to dismantle the Aristotelian universe. A crucial component of this was viewing the marks on the moon as craters, which evidently began the process of dismantling the claim that the heavens were perfect and completely spherical. Galileo’s moon drawings were published in Sidereus Nuncius in 1610. He would subsequently publish Letters on Sunspots in 1613, which would further help dismantle the conception of the heavens being perfect.

In sum, your question has a very long and complex history. The discover of Moon craters ended up being one of the most fundamental events in the history of astronomy. This is because instruments began to be deployed, like the telescope, which extended human senses and revealed new answers regarding the universe. Furthermore, following Galileo, new astronomers and philosophers began to question Aristotelianism through new ways.