Thomas Aquinas' idea of God being necessary is false. The Cosmos alone has no cause, needs no cause. The Cosmos alone exists, from within rather than from without, by its essence rather than a cause. This only the Cosmos as a whole is necessary, everything else is contingent might-not-have been. And only the cosmos is eternal, everything else within it has a beginning.

Reference

Kreeft, Peter (2014) Practical Theology: Spiritual Direction from saint Thomas Aquinas. San Francisco: Ignatius Press.