Aquinas on the necessity of God

The second way is from the nature of an efficient cause. In the world of the senses we find that there is a consequence of efficient causes, but we never find something that causes itself, and it is impossible to do because it would precede itself--which is impossible...Thus it is necessary to posit some first efficient cause which all men call God.

St. Thomas Aquinas, Summa Theologiae, part I, question 2.