It struck me that this sentiment is a very clear and succinct expression of how many atheists view theists. For that reason, I think it is worth consideration. I am actually going to purposefully avoid providing a follow up evaluation. I will let you draw your own conclusions. I just want to mine out what I see being expressed in this phrase so that we (both sides) can get a good look at it. It may seem strange, but I want to examine it from the end and work backwards.
NOTE: If you are an atheist, your gut reaction will probably be to take issue with what I've said below. However, I would encourage you to consider what I have said, and offer constructive thoughts where you think I have erred in my analysis. Please know that this post is not intended as an attack. Rather, in the interest of having better conversations, I think both sides need to learn to avoid insulting and/or overreaching rhetoric--which is mainly what I am aiming to expose here.
"I have hope that they simply haven't thought about it yet."
The author of this tweet hopes that the person they are talking to "simply" has not "thought about it yet." The presence of the word "simply" adds an air of triviality to whatever the pronoun "it" refers to. Even if we only had this second clause, we could conclude that whatever it is, it must be something painfully obvious. If only a person would simply take the time to think about it, they would most certainly come to a particular conclusion about it. The author believes the correct response is so obvious in fact, that the best hope for someone who disagrees is that they simply have not yet thought about it. So, what is it? The pronoun refers, although indirectly, to theism. If a person (a theist in this case) would only do the elementary task of thinking about theism, they would surely draw the same conclusion as the author.
"Every time I talk to an intelligent young theist."
The author says that there are many young theists who are actually intelligent. That might seem like a nice concession; unfortunately, it is offered in an underhanded sort of way in light of the second clause. But I will come back to that. First of all, why does the author specify theists who are young? That seems obvious. The assumption is that people who are older and/or have been theists for a long time are a lot less likely to recant their beliefs. That is probably a fairly safe assumption. But it sounds like (even if this wasn't the author's intention) that the author favors picking off the weak (those who have not yet thought about it) over dealing with the strong (those who have thought about it and remain theists). That is troubling. At that point, we come to the question of intelligence.
The author specifically singles out young theists who are intelligent. Again, at first blush, you might think that the author is saying one can be intelligent and a theist. Like I said before, that would be a nice (and surprising) gesture. By the way, I would offer as much to my atheist friends. I think you can be very intelligent and an atheist. Anyway, the context won't allow that here, at least not for very long. The author treats the intelligence of the theist as something wholly separate from his or her beliefs. Intelligence is something to be used to counteract his or her errant views before they really take root. In the author's view, theism and intelligence can only coexist up to the point that the person actually takes the time to think. One will eventually kill the other off. After that, if they remain a theist, there is no hope--that they are either too stubborn for their own good and/or not intelligent enough to come to the right conclusion. The latter seemed to be the implied sense, because, (according to the author), if they were intelligent and simply thought about it, they would be an atheist.