Thomas Aquinas From "Summa Theologica"

The Bible does not tell us, but Thomas Aquinas gave a classic answer in his book "Summa Theologica." Even while most Christians think his answer makes sense, please remember this is conjecture, not gospel. The Bible teaches that God created the angels, and like all of God’s creations they were "good," but that did not mean they were incapable of sin. To be incapable of sin, Aquinas wrote, the angels would need to be in a state of bliss that he defined as "seeing the essence of God" and being "confirmed in goodness by God’s grace.... No angel could of his own will turn toward that bliss unless aided by grace.... The fall of some angels shows that the angelic nature was not created in that state." His view was that the angels who chose to follow God demonstrated a willingness to receive God’s grace. Once this choice was made, these angels were blessed, confirmed in goodness, and lost the capacity to sin.

In Aquinas’s view the angels who sinned exercised their free will. At first they did not seek evil. Rather, they chose to do good but in a way that fed their selfish pride. Satan wanted to find bliss through his own actions rather than through God’s love and grace. This, then, was a major cause of his downfall.
When Did Satan Fall?
The Bible does not tell us. What we do know is that the angels were created before the Earth (Job 38-4-7). Satan fell before he tempted Adam and Eve in the Garden of Eden. The fall must have occurred somewhere between.

Read more at http://www.beliefnet.com/inspiration/angels/2006/09/angel-faqs-the-fall-of-satan.aspx?p=2#GI7OyBDSg3IqeHfZ.99h #angel