I believe many of our letters and numbers are Arabic in origin. The language itself developed with people, of course - then changed with each group as people lived in different geographical areas, then merged again, to some extent as people fought wars and moved into each other's territory. Now we have the mish-mash that is modern English, Spanish and all the other languages.
Languages change very quickly - I read that a hyphenated word had a life of around a year, before it became one word. So if we used mish-mash a lot, it would very soon be mishmash.
God, being non-existent of course, had nothing to do with it.
We did not "invent" them as much as we "developed them.
I won't give you the entire history lesson, but early forms of man had crude language, which like evolution formed from simple to more complex structures. The need to define, count, measure, barter, and share among ourselves necessitated the development of early mathematics.
These are thing that happened in pre history, when we were just little bands of nomads walking around and hunting and gathering. No one group of people can be attributed to this. Now the alphabet is different, this can be attributed to several groups around the mediterainian sea, most notably the phenecians.