To the chief Musician, A Psalm of David.
14:1 The fool hath said in his heart, There is no God. They are corrupt, they have done abominable works, there is none that doeth good. (KJV)
All holy books of every religion seek to do two things: Convince their religion's believers that they are right, and denigrate anyone who is not one of their religion's believers.
Atheists are targets of all religions. They stand against the notion that you must have that god to be a productive and happy member of society. Note the very claim in the verse from Psalms: They are corrupt, they have done abominable works, there is none that doeth good.
That quote from Psalms (the book that Christians who claim they are under a "new covenant" quote a lot, despite the fact Psalms is from the old covenant) of calling people "fools" is backed up by statements in the New Testament attributed to Jesus: Ye fools and blind. Matthew 23:17, 19; Ye fools. Luke 11:40; O fools, and slow of heart to believe. Luke 24:25. There are more attributed to Paul.
Jesus himself contradicts Jesus in the "Sermon on the Mount," however: Matthew 5:22 But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire.
The Sermon on the Mount deserves a deconstruction all on its own on why it is not the fluffy wonderous loving sermon claimed by Christians, so I'm not going to reinvent the wheel. Matt Dillahunty, former Baptist seminary student and current member of the Atheist Community of Austin has already done that two years ago. His deconstruction of the sermon and why it is not a loving view of humanity can be found at Youtube (36:00).
The real issue of claiming "atheists are fools," however is a straight up ad hominin attack. Since the position of atheism cannot be assailed (atheism has no position, it is a lack of belief), it instead attacks the messenger. The phrase is a logical fallacy. It should be addressed as both.