Ron, which dictionary do you like? Webster's? the OED? Funk and Wagnalls?

Agnosticism and atheism do overlap. You can be both. There are those who declare themselves to be so. The only difference is in a matter of degree.

It is unclear just what Einstein actually believed. He has said many things which comport with the views of an atheist.

As to arrogance, Christians are notable for their arrogance in the denial of anything except what they believe which is true of most religious believers. The defense of religion as being anti-atheist is in itself an arrogant assumption. It seems that religionists could use a strong dose of humility before they start in on atheists.

You say, "Atheism assumes more certainty than is justified." A lot has to do with who evaluates what is just and for what reason. People were put to death historically because religious leaders had more certainty that was justified, if we are to talk about justice.
The bloodletting of religion was done by religionists who had "more certainty than is justified".