As an agnostic, I’d like to paraphrase for how I see it:

“Aim at heaven and you will likely be disappointed. Aim at earth and appreciate the reality and beauty of this world.”

As an atheist, I would paraphrase as:

“Aim high and you will make progress. Set low standards and you’ll never get anywhere.”

He’s saying that a life lived with a view toward long term consequences works out better than one that seeks to optimize near term benefits. He uses Christian language because this was from a book on Christianity, but the principle is a sound one. It’s the gist of the famous Stanford marshmallow experiment. People with the ability to delay gratification tend to have better lives.

Even as an agnostic, I don’t have a problem with the quote. Think like an astronaut and it’s crystal clear.

I like @syz‘s version best. I do think, however, that the “heaven” part of the original statement could be considered metaphorical; “reaching for heaven” is like “reaching for the stars.” I think the “Earth thrown in” part is was threw me off of Lewis’ quote, not the reference to heaven.

I would rephrase it as “Aim at heaven and you might get hit with the bullet.”