Divine calling and regeneration stand in the closest possible relation to each other. With respect to regeneration several points deserve consideration:
a. Its nature. The word ‘regeneration’ denotes that act of God by which the principle of the new life is implanted in man, and the governing disposition of the soul is made holy. It designates the new birth or the first manifestation of the new life. It is a fundamental change in the principle of life and the governing disposition of the soul, and therefore affects the whole man, I Cor. 2:14; II Cor. 4:6; Phil. 2:13; I Pet. 1:8. It is completed in a moment of time, and is not a gradual process like sanctification. Through it we pass from death into life, I John 3:14. It is a secret and inscrutable work of God that is never directly perceived by man, but can be known only by its effects.
b. Its author. God is the author of regeneration. Scripture represents it as the work of the Holy Spirit, John 1:13; Acts 16:14; John 3:5, 8. Over against the Arminians we maintain that it is exclusively the work of the Spirit of God, and not in part the work of man. There is no co-operation of God and man in the work of regeneration. Regeneration is a direct and immediate work of the Holy Spirit. The Holy Spirit is able to regenerate a soul apart from the Word, but He is pleased to use the Word (ames 1:18 and I Pet. 1:23).
c. Its necessity and place in the order of salvation. Scripture leaves no doubt as to the absolute necessity of regeneration, but asserts this in the clearest terms, John 3:3, 5, 7; I Cor. 2:14; Gal. 6:15. This follows from the fact that we are by nature dead in trespasses and sin, and must be endowed with new spiritual 1ife, in order to enjoy the divine favor and communion with God. The question is often raised which of the two is first, calling or regeneration. In answer to this it may be said that in the case of adults external calling usually precedes or coincides with regeneration. Acts 16:14, “And a certain woman named Lydia, a seller of purple, of the city of Thyatira, one that worshipped God, heard us (external call): whose heart the Lord opened (regeneration in the restricted sense) to give heed to the things which were spoken by Paul (internal call).”
[From Berkhof Summary of Christian Doctrine]