Real truth is not a matter of opinion, nor is it simply personal perception. Truth is not relative; without question truth can be known with certainty, absolutely. We can make these bold declarations, basically, because of God - God is a God of truth!
God is truthful by character; we understand the concept and importance of truth because of His character. Without God there is no standard for truth. He is identified as the "God of truth" (Ps. 31:5), the very "Spirit of truth" (Jn. 15:26), so much so that He "cannot lie" (Tit. 1:2; Heb. 6:18). His very "word is truth" (Jn. 17:17), and it is that truth that is key to our forgiveness, redemption, salvation and eternal life (Jn. 8:32).
Many people find truth, at least in given circumstances, to be inconvenient. It's not what they want. They don't want to be constrained by the truth. As a result, they try to bend and reshape it to fit their desires at that moment. If they shout loud enough and long enough many people will come to believe their version of the truth. In reality, declaring falsehood to be truth does not make it so, and grave danger awaits those who practice such deception. Isaiah warned "those who call evil good, and good evil" (Is. 5:20).
Truth is truth by reality, not by consensus. Truth is truth whether or not anybody likes it or believes it. Cold, hard facts - evidence - determine real truth. People frequently deny truth, even call error truth. Sometimes it's out of ignorance, never having been presented accurate information. Often, however, even when presented the undeniable facts of a matter, they still deny the facts and hold onto error.
God has not left us without warning or instruction as to the importance of knowing and properly responding to the truth. One day we'll all face the judgment scene (2 Cor. 5:10) and be judged "according to truth" (Rom. 2:2). Since His word is that body of truth by which we'll be judged, we need to diligently study it in order to be prepared for that day (2 Tim. 2:15). Those who "do not obey the truth" will experience God's wrath (Rom. 2:6-9). We cannot expect to live in error in this world and live with God in heaven - God is a God of truth...