The word "God" is used as a name, so it's a similar thing. The capital letter in this word is also reverential. Many people will capitalize any word that stands for "God". They will write:
There is one God and everything has its beginning in Him.
We are at grave risk of starting a religious debate, but I agree with freezeframe: "that is a choice, not a necessity". People who believe in a being they call "God" will indeed write, "There is one God and everything has its beginning in Him". They are welcome to do so, but that does not mean that anyone else has to. In that sentence, the G-word is not a name. If it is reverential, there is no need for those who do not feel any reverence to feel obliged to show it.
Soothingdave made a number of points:
1. God is capitalized because the word functions as the name of this particular divine being. If I say "Zeus is a Roman god" I capitalize the god's name (Zeus) but not the description "god".
When the word functions as a name, then I agree.
For that matter, we normally capitalize "God" when referring to the Judeo-Christian, monotheistic God (as opposed to, say, Greek or Roman gods.)
That was the case when most native writers of English were followers of Judaeo-Christian beliefs, and is still the case for such believers. Soothingdave's 'we' does not include those who are not such followers.
I am describing the common usage that I am familiar with. If others choose differently, that is their prerogative. I thought a learner of the language should know that it can be considered disrespectful to not capitalize "God."
It may be considered disrespectful by those who consider it important, but not by others.
If we can tell learners that many prefer gender neutral words like "firefighter" over "fireman," in order to avoid offending some people, we can surely tell them what the norm is on this subject.
As a large number of people still capitalise the 'g', I agree that learners should know that some people are offended by a lower case 'g'. However, this is not a good analogy. The selection of gender-neutral words appears to be on the increase because many people are consciously trying to avoid what they now see as a use of language that (possibly unintentionally) predisposes some to view masculine forms as inplying the superiority of male beings. For similar reasons, some people are now using the lower-case 'g' for 'god' in order to avoid any implication that the Judaeo-Christian view of a deity is superior to other views.