Often the words truth and belief are used interchangeably. Yet their meaning is very different.
Truth is a word that means “what actually is”. It is irrelevant what people believe to be true. Belief has no influence on the truth. There are some truths that nobody knows, about something that is. Yet it still is. Truth exists, whether people know it or not.
There are 2 main kinds of truth. Truths that are defined by people, and truths that are defined by nature. We can point at a cat and say it is a cat. This is true because it has been pre-defined. We can look at the earth and say it is spherical, not flat. This is a truth defined by nature – the earth is either spherical or it is not. We rely on evidence to decide what we believe to be true.
Belief is opinion. A belief is what we THINK is true. We can believe the earth is not a sphere, yet our belief does not make the earth what we believe it to be. The earth will continue to retain its shape regardless of what humans on its surface believe. Humans hope to know the truth of something, by observing and considering evidence. But humans cannot control the truth they do not define.
A person who believes the earth is flat will highlight certain observations made, and will not mention other evidence that may not support their belief. One who believes the earth is a sphere will point out evidence that supports their conclusion, and ignore whatever does not support their belief. Both groups cannot be correct about what the shape of the earth is. Both cannot be true.
In order to know the truths defined by nature, by the universe, we must rely on the scientific method to collect evidence, postulate and test theories. We cannot make an arbitrary definition. The universe will not change its nature to suit our demands.
With regard to the shape of the Earth, we can point to other evidence such as the earths magnetic field, star charts made from different locations, and the coriolis force (that causes tubs to drain in different directions in the northern and southern hemisphere) to help us decide what is true. These things combined with other evidence, make it increasingly likely that the truth is that the earth is spherical, and increasingly unlikely that it is flat. The likelihood that the earth is spherical is so strong that most of us accept that it is so, and dont think about it further.
There are many truths that are simply not yet known by humans, and people argue about that their belief is the one that matches the natural truth. Insufficient evidence is no impediment to opinion. And belief is opinion.