Athanasius and God from God
For Athanasius the doctrine of the eternal generation of the Son speaks of the eternal begetting of the Son in the inner life of God by necessity, not will, that resulted in "God from God," a "true offspring," "one in being" (homoousios) with the Father.
Kevin Giles, The Eternal Generation of the Son: Maintaining Orthodoxy in Trinitarian Theology (Downers Grove, IL: InterVarsity Press, 2012), 5