This is a vast subject. We will only consider truth as you have referred to it. I suppose the most basic definition of truth would be: the conformity of the intellect with what the thing perceived actually is. This would be objective truth. In our culture many want to make such truth relative. “You have your truth and I have mine.” Such is not truth. If one’s perception of something does not conform to what it actually is, then one is in error—no matter how convinced one is and certainly no matter how one feels about it. People who are colorblind are not seeing all the true colors before them.
Their perceptions are distorted, even though they are not aware of it. When I make a statement about something that does indeed actually exist, then such a statement could be called a truth. Any statement that would contradict that statement would then be an untruth. If such an untrue statement is deliberately made in order to deceive, then that statement is called a lie. This is pretty basic stuff. Unfortunately, there is so much dishonesty in our society about the very nature of truth that many are confused. Since God is the source of all that is and knows his creation perfectly, he is the fullness of all truth.