There are two types of atheists which might meet the requirements of this question:
1. The "never was member of a faith community" atheist: In this case they just show up at the door of whichever Christian sect they may want to join and they will be provided with instruction.
2. The "once were Christians but left the fold and want to return" atheists: Many atheists have become such after discarding Christianity as a non-starter, not because they haven't heard the Christians "good news" but because they've thought it through. Either they change their mind and want to rejoin, in which case they just show up at the door of whichever Christian sect they may want to join and they will be provided with instruction, or well meaning friends or family decide to pressure them to rejoin. In that case it is like approaching a divorced person to explain all the good points of the abusive spouse they have left.
There have been many. If you count children, then every Christian was once atheist, since babies are not born believeing in a god - they have to be taught that.CS Lewis is one of the better known people who have become Christian after being atheistic.
christian
No he is an atheist
An Atheist.
no, he is a christian
No, she's a Christian.
And
No Atheist
No. He is an atheist.
No, she is an atheist.
Christians
Atheist