Often conflated. Certitude commonly refers to the attitude or state of mind, while certainty is correlative to the truth, the "condition of the evidence of a proposition." (As the Catholic Encyclopedia says.) Oliver Wendell Holmes said wisely, "Certitude is not the test of certainty. We have been cock sure of many things that are not so." That is not to say that certitude is always an improper state of mind. If something is clearly true, is beyond doubt, then certitude is the appropriate response.
More from the Catholic Encyclopedia: "The proper test of truth is evidence, whether the evidence of a truth in itself or by participation in the evidence of some other truth from which it is proved. Many truths, indeed, have to be accepted on authority; but then it has to be made evident that such authority is legitimate, is capable of knowing the truth, and is qualified to teach in the particular department in which it is accepted. Many truths which are at first accepted on authority may afterwards be made evident to the reason of the disciple. Such in fact is the ordinary way in which learning and science are acquired."
(worth reading the whole article here:) http://www.newadvent.org/cathen/03539b.htm
Certaintists need to address these arguments:
1. Certitude is dangerous for the world. As stated by Bertrand Russell "The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts."
2. Certainty is science, nothing more or less. (there is no moral certainty.)
3. There may be truth and thus certainty, but our ability to be certain (certitude) is impossible. Therefore, anyone who claims certitude is deluded.