It is also frequently asked what our belief must be about the form and shape of heaven according to Sacred Scripture. Many scholars engage in lengthy discussions on these matters, but the sacred writers with their deeper wisdom have omitted them. Such subjects are of no profit for those who seek beatitude, and, what is worse, they take up very precious time that ought to be given to what is spiritually beneficial.
What concern is it of mine whether heaven is like a sphere and the earth is enclosed by it and suspended in the middle of the universe, or whether heaven like a disk above the earth covers it over on one side?...
Hence, I must say briefly that in the matter of the shape of heaven the sacred writers knew the truth, but that the Spirit of God, who spoke through them, did not wish to teach men these facts that would be of no avail for their salvation.
St Augustine was addressing those who thought that the shape was a doctrine. That's why he used the phrase: "what our belief
must be". He says it's a waste of time, when viewed from the angle of doctrine or sanctity, because the topic concerns neither.
But it's inaccurate to say St Augustine didn't care, or thought it was a waste of time from a curiosity/science standpoint. He wrote extensively about the topic in 3 different books. It seems he studied it for a long time, wrote about it, then stopped. Then later in life, he revisited the topic in 2 of his other writings, including "city of God". He flipped back and forth between Plato's flat earth/dome model and the the pagan-style sphere model.
Interesting that Plato was a very big proponent of the natural law, while most other greek pagans were immoral pedophiles.