So, what is the difference between “belief,” “opinion,” “fact,” and “truth”?

First, let’s look at some dictionary definitions.

Belief: 
a feeling of being sure that someone or something exists or that something is true; a feeling that something is good, right, or valuable;  a feeling of trust in the worth or ability of someone; a state or habit of mind in which trust or confidence is placed in some person or thing; something believed; especially :  a tenet or body of tenets held by a group

Believe: 
to accept or regard something as true

Opinion:
 a belief, judgment, or way of thinking about something : what someone thinks about a particular thing;
belief stronger than impression and less strong than positive knowledge

Fact: 
something that truly exists or happens : something that has actual existence; a true piece of information; something that has actual existence; an actual occurrence; a piece of information presented as having objective reality

Truth:
the truth : the real facts about something : the things that are true; the quality or state of being true;
a statement or idea that is true or accepted as true
b :  sincerity in action, character, and utterance
(1) :  the state of being the case :  fact (2) :  the body of real things, events, and facts :  actuality (3) often capitalized :  a transcendent fundamental or spiritual reality
b :  a judgment, proposition, or idea that is true or accepted as true 

Most secular people today, and also many Christians, think that beliefs and opinions are identical. They also often think that facts and truth are the same thing. But there are very important distinctions between these four things.

A belief is something that is held to be true. It is distinct from an opinion in that it is often, especially in religious connotations, a tenet that regards the universal truth about objective reality. 

An opinion is a personal evaluation of someone or something that assigns a value or critique of that thing, such as a movie review.

A fact is a statement, usually about physical reality, such as a “scientific fact.” It is also often an accurate statement of events that have occurred.

“Truth” is the way things actually are, whether we like it or not. For example, either there is a God or there is not. One is true and the other is false. It cannot be both true and false that God exists. To assert that this is the case is to destroy rationality.

Two Notes: 
I disagree with the two areas of modern bias in the dictionary quotes above. A belief is not a “feeling.” (This is an attempt of modern unbelievers to make all beliefs a matter of emotion. They are not.

An older version of the ~same~ dictionary defines” belief” as, “an assent of the mind to the truth of a declaration, proposition, or alleged fact, on the ground of evidence, distinct from personal knowledge. I’d say that hits the target.

Secondly, under, “Truth,” I don’t know what a “real fact” would be in contrast to a “fact.” There seems to be a subtle scepticism as to what a “fact” is, included in this phrase.

–Curtis Smale