Tradition
St. Cyril of Jerusalem
Catechetical Lectures
Book 11 › Introduction
On the Words, the Only-Begotten Son of God, Begotten of the Father Very God Before All Ages, by Whom All Things Were Made.
Hebrews 1:1
God, who at sundry times and in various manners spoke in times past unto the Fathers by the Prophets, has in these last days spoken unto us by His Son.
Source: Catechetical Lectures (New Advent)