When I say that it's impossible for anyone to be inwardly truthful without Jesus, that's exactly what I mean. Every time a statement of this type is leveled at the rest of the world by a Christian, the unbeliever usually responds with an appeal to not only every culture and individual who never heard the Gospel (both before and after Jesus' arrival) but also to their own definition of what truth is.
And please understand that I mean no disrespect and am presenting this as winsomely as I can. "That I may not seem as if I would terrify you by letters." (2 Corinthians 10:9) Says Paul.
Anyone and everyone can be factual. All people have the ability to give the correct answer when asked a question. Most people will tell you the time if you asked. This might be truth, but it's not the truth to which I am referring. I'm referring to the universal (there, I said it) bedrock on which existence stands. But any reference to the rightness or wrongness of something must include the way in which things are perceived. Forget adding "by the world at large" to the end of that sentence because appealing to a majority never solves anything. It's illogical on the grounds which I have laid for this discussion. If we're all agreed on the same thing but that same thing on which we all have agreed is incorrect, then we're all wrong.
"Before whose eyes Jesus Christ hath been evidently set forth..." (Galatians 3:1)
"For such an high priest (referring to Jesus) became us, who is holy, harmless, undefiled, separate from sinners, and made higher than the heavens;" (Hebrews 7:26, emphasis mine)
I find it interesting, when looking at the topic of truth from a neutral perspective, to see that there is this idea loose in the world, of Objective Truth. When I ask you the time and you pull out your phone, or you look at your wrist, and proceed to inform me that it's "quarter to ten", you've just been truthful with me. You've told me an honest answer to my query and we're both the happier for it. Thanks, by the way. But! Human beings amidst and amongst human beings and their mores of interaction is what creates subjectivity. Each person with reference to the other.
A rhetorical question: Can objectivity arise out of subjectivity? Two actually: Can the very concept of objectivity arise out of subjectivity?
But when Jesus walks the earth and says that He is the truth (see John 14:6) and that He loves us (in obeying His Father by living for us and dying for us—He proved it), a new idea has been introduced into the culture. And it then grows and infects and spreads like wildfire. Suddenly we have an idea that people (call them Christians) are touting, bandying about like some newfound fad. Only, in the two-thousand plus year history of this idea, it's only grown by leaps and bounds. But this doesn't make the idea of "Objective Truth" any truer, does it? But spreading alongside, almost as a defense tactic, is the corollary that says that we are unable to perceive this truth on our own. This is one of the main reasons Jesus came. Also to reiterate the fact that another invisible entity—one of evil—"the god of this world" as Paul identifies him, one who "hath blinded the minds of them which believe not" (2 Corinthians 4:4, emphasis mine) is loose and at large in this world as well.
People are sensitive. I'm not talking about emotional instability, I'm talking about an ability to detect. Vegetarians (or vegans) can tell if there is something in their supper they'd rather not eat. The body of anyone who has food allergies will certainly speak up if any reaction-inducing substance is in their food. I heard a story once of a woman who knew the very moment she got pregnant. I assume she felt from that moment on her child growing inside her. Beautiful. But these are all physical examples. If in any way you could bring yourself to believe that there are things in this world to which you are sensitive that have nothing to do with anything physical, corporeal, tangible, please read on.
Tell me why you decide to be honest. Why did you tell me the correct time? Are you hiding something? Did you tell me it was "quarter to ten" because you were trying to hide the fact that you are dishonest? An honest question, no? No, you weird paranoiac. I told you it was quarter to ten because it was 9:44am and I wanted to round up. Make it easier on me. And you. Now get away from me. Fair enough. But you'll never know if that person is more truthful than that. Nor, really, should you care. "A fool uttereth all his mind: but a wise man (and woman) keepeth it in till afterwards." (Proverbs 29:11) The reason someone might feel something's amiss in spite of the fact that they got the correct answer is that they're either truly crazy—on something. Or they're onto something. The reason why we desire truth and honesty and rightness has nothing to do with our physical heritage, it has to do with the fact that we are created in God's image.
"And God said, let us make man in our image, after our likeness: So God created man in His own image, in the image of God created He him; created He them." (Genesis 1:26a, 27)
Because you have accepted Jesus, what happens—I'm just going to tell you—is that His light gets down to the very depth of your being. And you see yourself in a different way. You see yourself with reference to Him. This is what the Bible said would happen. You see truth as Jesus sees it. Truth becomes something with a capital "T". "Every spirit that confesseth not that Jesus Christ is come in the flesh is not of God." (1 John 4:3) No, it didn't say that renewing your mind to His truth wouldn't be a struggle. That part is excruciatingly* hard at times as now, you by your very (new) nature run counter to "the god of this world". The father of lies. What you have now is a new nature based on the truth that Jesus loves you and that you believe (and know) it. The moment you believe, God renews your sight to His truth. The god of this world has no choice but to spit you out.
"I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me and gave Himself for me." (Galatians 2:20)
Now! Now, your capacity for veracity has been expanded. Overhauled. Torn down and rebuilt. Now you can see the forest for the trees. You can see the sun behind every cloud (trust me, it's there). You can see what John's saying when he says "He (and she) that hath the Son hath life; and he that hath not the Son hath not life." (1 John 5:12)
"And that, knowing the time, that it is high time to awake out of sleep: for now is our salvation nearer than when we believed." (Romans 13:11)
*Excruciating, the root of which is the same as crucify.