What is atheism?
 Atheism is not a religion in the sense that Christianity, Islam, and Judaism are. Conventional religions usually include a belief in the nature of deity, other religious beliefs, a moral code, religious rituals and sacraments, and a membership in a religious community. Rather, an Atheist's belief system is confined to one factor: the existence or non-existence of a deity. He or she will have a personal moral code. However, it would generally be derived from purely secular considerations, and will be unrelated to any religious texts.
Most Atheists have analyzed the available material evidence for the existence of one or more deities and have concluded that there is no real evidence of the existence of gods or goddesses. They generally believe that the universe, Earth and its life evolved by perfectly natural processes. They see no evidence of intervention or guidance by a supernatural entity. They generally feel that ethical and moral systems governing human behavior can be developed without reference to any code of behavior of allegedly divine origin. Other Atheists are people who have simply never been exposed to belief in a deity or deities and therefore have no belief in them.
Tolerance
More and more, religious people are objecting to what they call "intolerance" on the part of irreligious atheists who are critical of religion, religious beliefs. Religious people insist that atheists are being intolerant and rather than criticizing or mocking religion, atheists should become more tolerant of religion. Liberal democracies place a high value on tolerance, so this sounds at first like a reasonable request but it's not because of how "tolerance" is being defined.
Tolerance is not a simple concept which either is or is not present; instead, it's a complex concept with a spectrum of possible attitudes. It is thus not only possible for a person to be "tolerant" of some idea, thing, or even person in one way yet not another, but it is in fact the norm. While it might be reasonable to expect tolerance in one sense, it's not necessarily reasonable to also expect tolerance in another.
My opinion: is that there should be tolerance between everyone,
atheists should be tolerant to religion, and religious people should be tolerant to atheists.