In AD&D 2nd Edition, when a wizard wakes up after a long rest they are required to study their spelltome to fix the magic they will be using for the day. When a priest wakes, they pray to their deities for the blessings they want, and fix their spells that way. What makes a wizard a wizard and a priest a priest though? Wizardly Priests and Priestly Wizards can interchange their spells between both. This is where confusion is most often times found, and it can be confusing for other players who are trying to understand mechanically why these characters can perform as such.
It is important to define a clear line between the two, but understand how the mechanic works for characters who walk the line. The character does not get double the spellpoints to allocate, so how do you decide how much goes to each?
What matters the most here is the source of where the power is coming. Arcane magic must come from some sort of store of arcana, some source that is on the same plane as the caster. It could be an item, a place, or even an event of some kind. In my own homebrew game world, I use what is called the Well of Magic. The Well of Magic is a source on my Prime Material that all arcane magic is sourced from. If your from the Prime Material, this is where you are drawing your magic.
Divine magic is much different, in that the source of the power comes from a being, entity, or outsider. A priest who prays to this intelligent omniscient source is granted a very small portion of influence as asked for, and allows the priest to cast their prayed for spells.
Should a player get to pick from both Arcane and Divine magics, they will use their maximum spell point allocation between the two, studying the spells they have access to, and praying for the blessings they desire. The primary difference between the two kinds of magic comes down to the source of where it is coming from.
To conclude, Arcane Magic's source comes from a place or thing from the Prime Material, and Divine Magic comes from a being, entity, or outsider with influence outside of the Prime Material.
