There are many modern religions and philosophies that say that truth is whatever you feel it to be.  That very explanation goes against the definition of truth itself. Truth itself is the truth no matter what anyone thinks. For example, nobody has to believe in the law of gravity, but that unbelief does not keep gravity from holding man on the ground. If a man is silly enough to say, "I don't believe in gravity, so I can jump off this bridge." He is still going to fall to the earth anyway. Likewise, the truth is ALWAYS the same whether some, all, or none believe in it.  

Also, the truth does not change over time. There are things that change over time, but something absolutely true does not change. Perhaps that is what the world does not understand the most.  Truth is an absolute, and God (Absolute) is the Author of it. So, now that we understand what truth is not, what is it? Christ says that He is the way, THE truth, and the life. (John 14:6) We are told that He is FULL of grace and truth. (John 1:14) If He is something and full of it, then there is nothing in Him that is in error.  

He cannot lie, because that would go against His very nature and existence. So, if Christ is truth, then the things that He did are absolute as well. If He died for us, then there is nothing that can be done to unravel that work. For example, death is absolute.  Unless the Lord returns first, all of us will die a natural death. Since that does not change, nothing can stop that from occurring.

 We try to keep ourselves in good condition, see the doctor when we are sick, have surgery if the situation calls for it, but nothing (short of the Lord) can stop us from dying. Likewise, since there is nothing higher than God, nothing shall keep Him from His purpose that He has in His Son. Therefore, we can boldly say that nothing shall separate us from the love of God. What is truth? It is Jesus Christ Himself.