Summa contra Gentiles1.80 Chapter LXXX. That God necessarily wills his being and his goodness

From what has been proved above it follows that God wills necessarily His being and His goodness, and that He cannot will the contrary.

For it has been shown that God wills His being- and goodness as principal object, which is the reason of His willing other things. Wherefore in everything willed by Him He wills His being and goodness, just as the sight sees light in every colour. Now it is impossible for God not to will a thing actually, for He would be only potentially willing; which is impossible, since His willing is His being. Therefore it is necessary for Him to will His being and His goodness.

Again. Whoever wills, of necessity wills his last end: thus man of necessity wills his own happiness, nor can he will unhappiness. Now God wills Himself as last end, as stated above. Therefore He necessarily wills Himself to be, nor can He will Himself not to be.

Moreover. The end in matters of appetite and action is as an undemonstrable principle in speculative matters: for just as in speculative matters conclusions are drawn from principles, so in active and appetitive matters the reason of all things to be done or desired is taken from the end. Now, in speculative matters, the intellect necessarily assents to the first undemonstrable principles, to the contraries of which it can nowise assent. Therefore the will necessarily adheres to the last end, so as to be unable to will the contrary. And thus, if the will of God has no other end than Himself, He necessarily wills Himself to be.

Again. All things, in as much as they are, are like to God, Who is being first and foremost. Now all things, in as much as they are, love their own being naturally in their own way. Much more therefore does God love His own being- naturally. Now His nature is per se necessary being, as was proved. Therefore God necessarily wills Himself to be.

Further. Every perfection and goodness which is in creatures, belongs to God essentially, as we have proved above. But to love God is the highest perfection of the rational creature: since by so doing man is, in a way, united to God. Therefore this is in God essentially. Therefore He loves Himself necessarily, and so He wills Himself to be.