I'm sure he includes me in that category, Clay, so he's not talking just about atheists.
I can speak only for myself. I am always careful to distinguish between dogmatic and non-dogmatic Christians. There are a good many Christians in this country who:
1) Do not believe in a Hell for unbelievers;
2) Do not believe in the inerrancy of the Bible; and
3) Do not believe that Christianity is the exclusive source of the truth.
So I always try to make it clear that what I'm against is dogmatism, not Christianity. If I make a particular target of Christian dogmatism rather than that found in other religions, which is often just as bad, it's because this is America, a majority Christian nation, where Christian dogmatists are a serious problem, and, say, Muslim dogmatists only become difficult when they fly airplanes into buildings.
Why oppose dogmatism? I can give you Jefferson's answer: "For I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man." Might be worthwhile to take a look at the full letter which Mr. Jefferson wrote to my collateral ancestor, Dr. Benjamin Rush:
Thomas Jefferson: I Have Sworn Upon the Altar of God, 1800
Thomas Jefferson said:
I promised you a letter on Christianity, which I have not forgotten. On the contrary, it is because I have reflected on it, that I find much more time necessary for it than I can at present dispose of. I have a view of the subject which ought to displease neither the rational Christian nor Deists, and would reconcile many to a character they have too hastily rejected. I do not know that it would reconcile the _genus irritabile vatum_ who are all in arms against me. Their hostility is on too interesting ground to be softened. The delusion into which the X. Y. Z. plot shewed it possible to push the people; the successful experiment made under the prevalence of that delusion on the clause of the constitution, which, while it secured the freedom of the press, covered also the freedom of religion, had given to the clergy a very favorite hope of obtaining an establishment of a particular form of Christianity thro' the U. S.; and as every sect believes its own form the true one, every one perhaps hoped for his own, but especially the Episcopalians & Congregationalists. The returning good sense of our country threatens abortion to their hopes, & they believe that any portion of power confided to me, will be exerted in opposition to their schemes. And they believe rightly; for I have sworn upon the altar of god, eternal hostility against every form of tyranny over the mind of man. But this is all they have to fear from me: & enough too in their opinion, & this is the cause of their printing lying pamphlets against me, forging conversations for me with Mazzei, Bishop Madison, &c., which are absolute falsehoods without a circumstance of truth to rest on; falsehoods, too, of which I acquit Mazzei & Bishop Madison, for they are men of truth.
Dogmatism is the enemy of truth, because it is the enemy of thought and discovery. It is also, in the end, the enemy of God, for God is discovered, not taught, found, not imposed, and awakened within, not communicated from without. Reason enough to oppose it, I think.