I was wondering if this word is in anyway related to some ancient diety or religion, if so which ?
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The entry in Etymonline that Mahnax posted is correct, as far as it goes. But it turns out that elder the tree and elder = older come from different Proto-Indo-European (PIE) roots, according to the AH Dictionary of PIE Roots.
Elder the tree (English elderberry) comes from the PIE root *el-² (i.e, the second root in the PIE dictionary that has the shape el- -- the asterisk in front of the root is to show that it's reconstructed, not attested (PIE was spoken long before writing was invented. This root meant 'red, brown'; other English words derived from it include elm, alder, elk, auk, hellebore, and eland.
Elder the comparative form of old, on the other hand, comes from the PIE root *al-³, which means 'to grow or nourish'. A short list of other English words from this root includes alderman, old, alto, haughty, hawser, altitude, enhance, adult, alumnus, alimony, proletarian, and abolish.
Elder = older is an example (as Etymonline says) of Umlaut. Umlaut means changing a back vowel like a or o to a front vowel like ä or e in anticipation of a front vowel in the next syllable. This is still a productive process in German, where alt, the word for 'old', has a comparative älter (pronounced /elter/). But it's no longer productive in Modern English, though it's also responsible for the plural mice from mouse (originally /mise/ from /mus/ before the Great Vowel Shift).