Atheism is defined as disbelief or lack of belief in the existence of God or gods and the devil. And just like Christianity is a belief, a belief in “God as the father, the son Jesus Christ and the Holy Spirit”, a complete belief in the Bible, a belief where you are not supposed to question the Bible.
Atheism is also a belief to reject the existence of any deity, a belief as the only other option when we stop accepting the Bible or any other religious book.