One way of thinking about truth and falsity is by asking questions on the probability of a given belief. One just has to develop a system of thinking through the likelihood of a given belief.
Geisler and Rhodes in “Come let us Reason” lay some general guidelines on probability which I include below from page 134.
99% – Virtually Certain – Overwhelming evidence in its favor. Things like the law of gravity fit here.
90% – Highly Probable – Very good evidence in its favor. It is highly probably that no two snowflakes that you see in your yard are alike.
70% – Probable – Sufficient evidence in its favor. Most medicines have to pass this test.
50% -Possible – What are the chances that your team will win the coin toss.
30% – Improbable – Insufficient Evidence in its favor. At this point, no one believes it except the few that it worked for.
10% – Highly improbable – Very little evidence in its favor. Like the theory that Jesus spent his early years studying with a Hindu Guru.
1% – Virtually Impossible – Almost no evidence in its favor. The existence of unicorns is at this level.
Perhaps we could start with God’s existence. How probable, given the world that we live in, is the existence of God? At this point, atheists and theists will disagree. Yet, there seem to be good reasons to think that the features of the universe we live in are more probable given God’s existence than his non-existence. The Philosopher Michael Peterson (Christian) does this continually against the Philosopher Michael Ruse (Atheist) in their “Science, Religion, and Evolution” book. I highly recommend the read. If the likelihood of the universe developing, biological adaptation, evil and goodness, free-will, and more are more likely given a theistic view of the world, then God’s existence is more probable than his non-existence.
However, there are other religious beliefs that are not likely. Namely that this world is held up on a turtles back, or the universe is eternal, or that Joseph Smith really did re-start and purify the church. If you can show that the likelihood of a given belief is higher than 50%, you are in a ok place to defend the belief. The higher up the line you go with the percentages, the higher the likelihood of the belief.
It might even help us to examine various beliefs that we hold about ideas, people, or institutions against various evidence we are aware of to figure out the basic probabilities or likelihoods of their in fact being true.