What scientific discoveries greatly weakened religion and the case of God ?

Science and religion are often compatible, I know of some Hindu thinkers (for example) who say scientific knowledge is to be taken as truth and religious truth should not contradict it - just that this scientific knowledge cannot explain the whole mysteries of the cosmos. You might be aware of "the god of the gaps" and arguments like that, or that God somehow created the universe using the laws of physics as we understand them. Historically, scientific thought and religious thought were often united and people pursued science and philosophy due to attempting to understand God (like many Islamic scholars in the 7th century or like Renee Descartes who ultimately sought to prove the existence of God by pure reasoning). Science as a complete system of belief without some religious aspect is actually a fairly recent phenomenon that likely had very little to do with any particular scientific discovery.

Indeed, science can do very little to explain why things happen. It's great at explaining how - e.g. science is great at explaining how fire burns or how a calculator can display an answer but it can be iffy on why. Now, "why" fire burns is probably more of a malformed question like what's north of the north pole but we're human, we like to ask why and seek purpose. Meaning makers.

The decline of religiosity wasn't really driven by science showing biblical stories weren't real, it's a process driven by material reality and class relations. Although many people considered themselves Christian or religious in the west, they were very Deist and didn't think God had much influence with the world apart from answering paradoxes like what was the primum movens etc.

Going further back, religion wasn't a choice or something to reason to - it was just your life and your community. In medieval Europe, you didn't really reason your way into a system of beliefs they all tied together into an economic system called the feudal mode of production. You just were a Christian and so was everyone you knew. Maybe some monks debated some esoteric aspects of theology but most people just lived their lives. This lasted for a while through to the Enlightenment and the emergence of capitalism in the 17th century. Except for some malcontents and rebels, people still didn't reason towards being Christian, say. It was just your life - more like a hangover from that older mode of production and social cohesion than something necessary to maintaining capitalism.

Fast forward to the actual decline of religiosity and rise of spiritual none-of-the-aboves and nothing-in-particular. This was a process started in the mid 20th century (not really in WW1 which was conceived often as a holy or religious war by the soldiers and officer class including miracles and appearances of angels and so on). In reaction to the rise of consumerism and individualism - now religion became a choice or affectation! This is where we start to see the irreligious begin their massive growth but especially by the beginning of the 00s. It's tempting to say Quantum Mechanics, GR, a scientific basis for cosmological origins like the big bang are responsible for the loss of religion - but in my view, these just coincided due to a third cause (that of economic changes and the settling in of mature capitalism).