I think there are very important differences.
Specifically if you are a christian (as an example) you already have a very clear roadmap set out for you as there are christian teachings, scriptures etc etc. An atheist is not like that - atheism is defined as a lack of something (in this case a belief in god) so there is no clear road map, anymore than there might be a clear roadmap for people who do not believe in Thor or who aren't christians.
It is possible, of course that someone who does not believe in the christian god might have a clear roadmap (e.g. if they believed in the hindu gods or had a strong belief in socialism through upbringing) but that again is different.
So what you implying is that some atheists might have a roadmap, but that wouldn't necessarily have any relevance to their atheism (christians can also have a particular political viewpoint etc etc) but there is no 'atheist roadmap' in the manner that there is for religions.
Yes, there is no atheist manual - and my point was that atheists are a product of their nature/ nurture, similar to theists.
Regardless of the words in a Christian roadmap such as a Bible (translations vary between literal translations and dynamic equivalence) or even a talk given by a priest, the words have no impact on the brain until a person e.g. a Christian interprets the words according to their nature/ nurture based on all the input they have from their environment/ culture that shape the moral values they hold. Hence, Christians disagree about ethical issues despite a roadmap.
The environment / culture for both theists and atheists, that influence the moral values they both hold, would have a lot of influence from Christianity as well as non-Christian and non-religious influences e.g. ideas from other religions as well as philosophical and political beliefs.
So I am saying there is not much difference since a theist's practice of their interpretation of religious words would be influenced by their individual personal moral values derived from both Christian and non-Christian inputs.