"at the beginning" implies a reference to a more specific point in time,where as "in the beginning" implies something occuring over a slightly longer period of time.They are often interchangeable,though.
At.是指的时间比较确切的时间点,In.指的时间稍长,但是一般能互换.
At the beginning (of) is used for the time and place when something begins.In the beginning (= at first) suggests a contrast with a later situation
At.指的是事情开始的时间和地点,In .（= at first 一开始）一般会有一个稍后的对比.
例句：
I would like to start with.lets start at the beginning
In the beginning god created man