By a consent almost universal the word regeneration is now used to designate, not the whole work of sanctification, nor the first stages of that work comprehended in conversion, much less justification or any mere external change of state, but the instantaneous change from spiritual death to spiritual life. Regeneration, therefore, is a spiritual resurrection; the beginning of a new life. Sometimes the word expresses the act of God. God regenerates. Sometimes it designates the subjective effect of his act. The sinner is regenerated. He becomes a new creature. He is born again. And this is his regeneration. These two applications of the word are so allied as not to produce confusion. The nature of regeneration is not explained in the Bible further than the account therein given of its author, God, in the exercise of the exceeding greatness of his power; its subject, the whole soul; and its effects, spiritual life, and all consequent holy acts and states. Its metaphysical nature is left a mystery. It is not the province of either philosophy or theology to solve that mystery. It is, however, the duty of the theologian to examine the various theories concerning the nature of this saving change, and to reject all such as are inconsistent with the Word of God.

Charles Hodge, Systematic Theology, Vol. 3, 6.