Therefore, why do we need to posit a distinct work of grace prior to an external Word, particularly when the New Testament typically relates the new birth to that Word? In my view, this distinction between regeneration apart from means and effectual calling through the Word is both exegetically untenable and theological unnecessary. Although the distinction between a general call and an effectual call (regeneration) can be easily sustained by exegesis, a further distinction between regeneration (unmediated) and effectual calling (mediated) seems to contradict the explicit references to regeneration by the Word cited above. (Michael S. Horton, Covenant and Salvation: Union with Christ, 238)
Effectual calling is the term used to describe God's special calling to the elect that will bring them into salvation. Regeneration is the term used to describe the internal change within a person that results in the person having a new heart so that he can respond in faith to the Gospel. Dr. Horton in his book however argues that we shouldn't have these two terms as two distinct works of grace. Utilizing speech-act theory, he argues that Gods effectual call itself creates the internal change in a person. He denies the idea of regeneration as an infusion of a new habit (habitus), seeing it as a vestige from medieval errors on salvation.
Traditionally, some of the texts concerning regeneration are passages like Ezekiel 36:25-27. It seems that the text is actually saying that God will put a new heart and put His Spirit in a person to change that person. This of course does not mean that any habits are infused into the person, for the Spirit of God is NOT a habit.
If regeneration is understood as an infusion of habits, then Horton's critique just might have a point. But if one takes the language of Ezekiel and thus of Scripture seriously, it seems that the focus in regeneration is empowerment by the indwelling of the Holy Spirit, not an infusion of any habit whatsoever. It is, to use Horton's preferred term, koinonia, through the Holy Spirit. Furthermore, since these "acts" are only distinct in terms of their logical flow in the Ordo Salutis, there is no necessity that the acts are in fact separate from each other. Effectual Calling and Regeneration can and do operate side by side even simultaneously, for the effectual call goes out to the elect whom the Holy Spirit regenerates.
Effectual calling and regeneration describes two concepts, and Horton's criticism against the term "regeneration" does not hold. Instead of removing a legitimate term, perhaps it would be better to understand the Ordo is a logical ordering of concepts, not a temporal order of separate acts.