Aquinas’s approach in each case by travelling down the via negativa, showing how both eternity (absence of mutability and so temporality) and unity (absence of division and thus composition) are in the first instance negative terms.
Divine Eternity
Eternity is a name ascribed to God from the starting point of a negation. It denotes first that God is not mutable or subject to change, and that there is no temporal succession in God.
Divine Unity
What does it mean to say that God is one?
For Aquinas, unity is a transcendental notion, like goodness and truth, which means that our terminology for “unity” is in some way co-extensive with our terminology for “being.” Whatever exists can be said to be one in some way.
Unity is an absence of division.
First, the affirmation of the oneness of God follows logically from the affirmation of his simplicity: there is no composition in God of the kind we typically find in creatures, of individual and essence.
Second, the unity of God follows from his infinite perfection.
Prodigal Catholic 