Faith

Faith is the substance of what we hope for, the evidence of what we cannot see, says the letter to the Hebrews. Faith is a disposition we characterize by the activity of believing it disposes us to, and that activity we characterize by the object believed in. Now believing is an assent of mind commanded by the will, so faith's activity relates to its object both as to a good willingly pursued and as to a truth mentally assented to. Moreover, being a theological virtue in which goal and object are identical, the way the object is faith's goal will correspond to the way it is its object. Now faith's object is unseen Truth itself and whatever else we assent to because of that Truth. So faith's goal is also Truth itself as unseen, that is to say unachieved yet hoped for. So Hebrews expresses the way faith relates to Truth as goal, the object willed, by saying Faith is the substance, or seed, of what we hope for, since what we hope for is to see openly the Truth we already assent to by faith. 

From: Summa Theologiae
          A Concise Translation
          Timothy McDermott

God's Will

To exist is actually to possess our own natural form; to know actually to possess the form of what we know. When things haven't got their natural form and perfections they seek them and do not rest till they have them; and in things lacking awareness we call this tendency towards their natural good their natural desire. In the same way things with understanding have a natural tendency to seek whatever they understand as good, not resting till they possess it; and this tendency we call will. So anything that has understanding has will, just as anything that has sense-perception has animal desire. We call will a desire or an appetite, through it doesn't only desire what it doesn't have but also loves and takes delight in what it does have. And this is how will exists in God, who eternally possesses the good which delights his will, since it is nothing other than his substance.

Summa Theologiae
A Concise Translation
Edited By Timothy McDermott

Existence of God

I have been recently reading a translated version of the Summa Theologiae by St. Thomas Aquinas.

St. Thomas has five ways to describe the existence of God.
The following is one of the descriptions I thought gave a good philosophical explanation of God.

"The first and most obvious way is based on change. We see things changing. Now anything changing is being changed by something else. (For things changing are on the way to realization, whereas things causing change are already realized: they are realizing something else's potential, and for that they must themselves be real. The actual heat of a fire causes wood, already able to be hot, to become actually hot, and so causes change in the wood. Now the actually hot cannot at the same time be potentially hot, but only potentially cold. So what changes cannot as such be causing the change, but must be being changed something else.) This something else, if itself changing, is being changed by yet another thing; and this by another. Now we must stop somewhere, otherwise there will be no first cause of the change, and, as a result, no subsequent causes. (Only when acted upon by a first cause do intermediate causes produce a change; if a hand does not move the stick, the stick will not move anything else.) We arrive then at some first cause of change not itself being changed by anything, and this is what everybody understands by God."
Summa Theologiae
A Concise Translation
Edited By Timothy McDermott