Eternal generation is “eternal and changeless" activity in the Godhead by which the Father produces the Son without division of essence and by which the second person of the Trinity is identified as an individual subsistence or modus subsistendi (mode of subsistence) of the divine essence.1
Another way to say that is God the Father generates the person of the Son by communicating to him the essence of the Godhead yet this generation is not a creation as if the Son (pre-incarnate Word) thus came into existence. This deals with the apparent relationship between Father and Son, which suggests a generation of the Son by the Father.
This generation would have to be necessary and not contingent. If there were a contingent, the existence of the eternal son would be dependent upon the Father and this would deny his eternal deity.
- 1. Shedd, W. G. T., & Gomes, A. W. (2003). Dogmatic theology. "First one-volume edition (3 vols. in 1)"--Jacket. (3rd ed.) (954). Phillipsburg, N.J.: P & R Pub.