Agnosticism is typically understood as maintaining the general premise that there are insufficient rational grounds for either accepting or rejecting claims like "God exists" or "God does not exist".
By contrast, theism affirms that God (or perhaps Gods) does in fact exist, while atheism affirms that no gods exist (the lacking belief definition seems muddy to me). Now, it seems fair to say that atheism can be applied to particular cases as well. A Christian, for example, positively believes that Allah does not exist, and is in that sense an atheist when it comes to Islam.
But it seems to me that if one is going to be an agnostic, one might as well be an atheist towards most religions given that these faiths seem to positively claim that there are rational grounds for accepting their beliefs. Christianity and Islam, for instance, are clear that we have been given all the rational signs we require for belief, and that deep down we know that God exists. Thus, a reasoned form of agnosticism ought to rule out these religions and collapse into a kind of atheism (with respect to these cases).
Is this a correct understanding of agnosticism and its relationship with world religions? Or am I misunderstanding something?