One thing that puzzled me in my early Christian years about God the Father and God the Son, was how the Son could be equal to the Father in every way. It seemed the very fact that he was called “Son” necessarily implied that he was inferior to his Father, in age if nothing else.
But St. Thomas argues:
The Son is necessarily equal to the Father in greatness. For the greatness of God is nothing but the perfection of His nature. Now it belongs to the very nature of paternity and sonship that the Son by generation should attain to the possession of the perfection of the nature which is in the Father, in the same way as it is in the Father Himself. But since in men generation is a certain kind of change of one proceeding from potency to act, it follows that a man is not equal at first to the father who begets him, but attains to equality by due growth, unless owing to a defect in the principle of generation it should happen otherwise. From what precedes (27, 2; 33, 2,3), it is evident that in God there exist proper and true paternity and sonship. Nor can we say that the power of generation in the Father was defective, or that the Son of God arrived at perfection in a successive manner and by change. Therefore we must say that the Son was eternally equal to the Father in greatness. Hence, Hilary says (De Synod. Can. 27): “Remove bodily weakness, remove the beginning of conception, remove pain and all human shortcomings, then every son, by reason of his natural nativity, is the father’s equal, because he has a like nature.”
St. Thomas Aquinas, Summa Theologica I., Q. 42, A. 4.
Human sons are inferior to their fathers — smaller, weaker, less intelligent (at least until they mature) — because men must proceed “from potency to act”. In other words, we don’t attain to the perfection of our nature all at once, but must acquire various qualities and abilities through growth and training.
It is not so with God, who does not arrive at perfection “in a successive manner and by change”, but was always perfect. In other words he always possessed all the perfections of his nature. He didn’t grow like a human baby from a little bit powerful to more powerful, and then eventually to all-powerful, but was all-powerful from eternity.
Since Jesus is God and possesses the same nature and essence, he too possessed all the perfections of his nature from eternity. Therefore the Son is equal to the Father in greatness.