For the first 2 decades of my life, my guiding light was the Christian god, as understood through the Bible. I thought of the Bible as a way to understand God, and I thought of God as the reason for being alive. I didn't have to find a purpose for life because it was handed to me: obedience to God. I didn't have to figure out what was the best choice to make because in most things the church or some elder in it was happy to tell me.
I worked hard to fulfill this purpose: I read the Bible cover to cover multiple times, attended church with great interest and took notes, often talking with the pastor after a sermon about something I was especially interested in.
Then my pastor said something in church that contradicted the Bible. He said that our greatest purpose was to "spread the gospel" -- so I went up after the sermon and asked "how do you reconcile this with Jesus saying that the greatest purpose is to love God, self, and others?"
He did not have an answer, so he attacked me verbally and told me that I wasn't really part of the church, that I hadn't done this or that and so I did not belong. He attacked me to the point where I cried (which I almost never did at that age), and when his wife came up and tried to stop him, he told her to shut up.
At that point I was done with that church, and I began to question the validity of many of the things I had accepted as true. I spent several years looking for a better church and evolving my understanding of God to include other religions. I still held that "love" was the greatest purpose, the one all humans share, and I tried to find others who believed that and tried to practice it in their daily lives.
I slowly came to realize that most people who would claim that love was their highest value were wrong. Most of the time they would choose to do things that were not the most loving action available to them, but instead would choose actions that made them feel good or made them look good to other people.
When I became educated about how systemic inequality works, I also came to realize that love without justice is worthless. So many well-meaning privileged people are against hatred or overt bigotry, yet because they are unwilling to do the work it takes to create justice, they reinforce oppression.
They want to express their sadness about injustice and give out hugs, but if you ask them to advocate for equal pay, to elect people of color to positions of power, to resist oppressive policies in their cities and organizations, or to consider the words they use or the assumptions they make that end up hurting people, they will be offended that you even asked. They will claim that "love conquers all" as a way to avoid the responsibility to create justice, which is the only thing that actually conquers injustice.
So I rejected "love" as the greatest purpose and embraced "justice" instead. Now, when I come to a decision point and have to make a choice, I consider everything I know about the effects my actions will have on others and I choose based on what is most likely to lead to a more just world.
This does not work without constant self-education on the effects of my choices. For example, I cannot make the most just choice about how to host an event without understanding how to provide access for the greatest variety of people. If I just tried to make a choice based on what felt right, I would be very likely to unintentionally harm or exclude some of the most vulnerable people in my community. I know that my own access needs are not the same as others' needs. So I must self-educate, and my greatest tool for that is the internet.
My guiding light now is justice, as understood through as many stories as I can find. If I had a holy book, it would consist of people discussing injustice through both statistics and through personal stories about racism, sexism, anti-trans and anti-queer attitudes, anti-intersex attitudes, ableism, classism, ageism, sizeism, capitalism, and any other form of systemic oppression. It is through reading these stories and learning about the overall societal impact of injustice that I learn how to aim my choices to create the greatest amount of justice that I can.