"Is Magica Finite"? 

The answer to this is, of course, "No". Although in very rare occasions the stored and ambient Magica of an area can be used up, much like air rushing in to fill a vaccuum, the Magica from the surrounding areas will fill the void.

What's interesting is that although we CAN create a Magica starved environment through the use of specialized wards, any Human entering into this environment will begin to fill it.

Although it is not a large amount, every Human "spills" Magica into their surroundings. It is theorized that every Human acts like a direct pinpoint to some great realm of pure Magic.

Humans are not the only source of Magica, but we are the greatest.

Demons are of course, the opposite. They produce no Magica and use it actively to stay alive.

The Priests state that Magica is a manifestation of the Lost God's love for Humanity.

Perhaps.

-Advanced Magica research and development course at the Grand School of Magica