Now, if you're a believer and you just read that, you're probably a bit boggled. I mean, I suppose you might feel insulted, but I'd bet that a lot of you are still stuck on, "He thinks what?" And possibly you're giggling just at bit at how comically wrong I am about religious folks.
That's pretty much the way nonbelievers feel when we hear (or read) things like this:
"As a Christian, its my position that God has revealed Himself to all mankind so that we can know for certain who He is. Those who deny His existence are suppressing the truth in unrighteousness to avoid accountability to God. It is the ultimate act of rebellion against Him and reveals the professing atheist's contempt toward God."
This is the same person I quoted last time, and in an odd way that's actually a compliment. Yes, I take issue - rather seriously - with his views, but he is at least reasonably friendly and quite well spoken. In addition, he is willing to put into words some attitudes that I frequently encounter as unspoken assumptions.
Like the last example, this is something that you should probably never say if you're a Christian talking to unbelievers, former Christians, or even believers of other faiths/religions. It displays the same fundamental arrogance as the last assertion I examined, but (for me, at least) it's so laughably misguided that I can't even bring myself to be offended by it. Still, as a favor to everyone, I'm going to break this one down, too.
So here goes. Why is this view a problem?
I know more about you than you do. This is another situation where someone has told you something about their life and/or their experience, and your response is to tell them that you know better. It's an arrogant assertion that you have the right and/or the knowledge to tell someone else what they really believe. As you might imagine, this is not an especially winsome or effective approach to sharing your testimony.
It's ridiculous. Look at that middle sentence again: "Those who deny His existence are suppressing the truth in unrighteousness to avoid accountability to God." Um, what? Seriously, have you thought about this? There's an all-knowing, all-powerful God. The fact of His existence is unmistakable. His judgement is inescapable. (That's what all-powerful means.) And yet, there is a significant portion of the human population who claims that He does not exist because they think this will... accomplish what, exactly? Allow them to avoid His judgement, even though He is all-knowing and all-powerful? That makes no sense.
No, if the nature and existence of God are as truly unmistakable as this sort of Christianity would have me believe, and if the consequence of unforgiven sin is an eternity of torment, then the only sensible response would be to do whatever it takes to obtain that forgiveness. Contrariwise, if people claim not to believe, then the obvious conclusion is that the nature and existence of God is not so obvious to some people as it is to others.
It denies any possibility of disagreement. If you insist that people only refuse to recognize God out of selfishness, anger, or rebellion, then you completely deny the possibility that people might have honest doubts or disagreements. This has (at least) two ill effects on your argument - first, for those of us who do have honest doubts or disagreements, it makes Christianity looks silly. Rather than providing witness for the Good News, it offers further evidence that Christianity doesn't know what it's talking about. And second...
It effectively ends the conversation. I might point out that the nature of God, and even the existence of God, is not at all apparent to me. This is true, but what's the point in trying to tell you that? You've just stated that anyone who says that is, by your definition, either lying or delusional.
At this point, you've lost any further opportunity to share the Good News. You can't address my doubts, because you don't believe that I actually have any. There's no point in telling me about how wonderful God is, or how insanely cool it is that Jesus died for my sins, because you already "know" that I'm in willful rebellion against Him. There's no reason for you to continue talking to me. And for my part, you've demonstrated that you're either unwilling or unable to deal with me as I actually am, so there's no reason for me to continue to talk to you.
My advice to evangelically-minded Christians? Take it easy. More to the point, take nonbelievers - or those who believe differently - at their word. I don't look at the world and immediately think, Wow, that's so incredibly wonderful, there must be a God, but I do look at the world and think, You know, if there is a God, he sure loves variety. And beetles. But definitely variety. And that variety extends to people, too.
So please, do us the courtesy of assuming that our beliefs - or lack of beliefs - are based on the best of our understanding. Trust us that we are being honest when we say that we do not believe, or that we believe something different. Deal with us as we are, instead of how you think we must be.
And when people disagree with you, don't assume that doing so is clear evidence of their moral failings.
 For unbelievers, it's a great relief if we don't have to listen to people insist that their understanding must be correct because it's based in scripture, even though it's laughably inaccurate when compared to observable reality - i.e. our own experience. For believers, it's a chance to avoid doing something that actively works against you when you're evangelizing, proselytizing, or otherwise sharing the Gospel. That bit in the Sermon on the Mount about how you're blessed when men revile you? Yeah, that only works if they're reviling you for the sake of Christ. If men revile you because you come across as a self-righteous twat, I'm pretty sure it doesn't count.
 ...for whatever reason...
 At best, it looks silly. At worst, depending on the interpersonal balance of power, it's directly threatening: you will be judged according to what I say about you, and what you say about yourself does not count. You are at my mercy.
 The usual next step in this exchange is for the would-be proselytizer to say something like, "I pray that God will soften your heart." I'm sure this is meant well, at least for the most part, but it seems to contain some rather disturbing implications. If I can't perceive God because my heart has been hardened, who exactly is responsible?
I can say with some certainty that if my consciousness continues after death, and if there are things I can do to ensure that it continues into paradise rather than torture, then I'd very much like to know about it. So I don't think I'd be the one hardening my heart. The alternative, of course, is that God Himself must be hardening my heart. While this is somewhat flattering - after all, Pharaoh got the same treatment - it also seriously calls into question the idea that God is just, let alone merciful.