Apropos of my posts in the past couple of weeks about Warfield, mediation, regeneration, and changing natures, here is a lengthy and very helpful quotation from Michael Horton’s The Christian Faith: A Systematic Theology for Pilgrims on the Way 
“The question at least among the Reformed is whether the effectual call is synonymous with regeneration or whether regeneration is a distinct and logically antecedent work of the Spirit. Earlier in the tradition the terms regeneration and effectual calling were used interchangeably. Regeneration (or effectual calling) is the Spirit’s sovereign work of raising those who are spiritually dead to life in Christ through the announcement of the gospel. Later, especially after extensive interaction with Arminianism, many Reformed theologians argued for regeneration as God’s act of infusing the habit or principle of life in those who are dead so that they will embrace the gospel when they are effectually called by the Spirit. Regeneration, then, became understood as a direct act of God, without any creaturely means, while effectual calling was seen as mediated through the preaching of the gospel. The special concern of those who embraced this distinction between regeneration and effectual calling was to guard the important point that regeneration (the new birth) is not dependent on human decision or activity but is a sovereign work of God’s grace. Even after acknowledging the impressive exegetical and confessional credentials of the older view, Louis Berkhof follows Charles Hodge in regarding the distinction between immediate regeneration and effectual calling through the Word as a useful one.
“I adopt the earlier view on exegetical grounds . . . .
” Although we must distinguish regeneration from conversion, I do not see the basis for a further distinction between regeneration and effectual calling. Scripture indicates that we ‘have been born again . . . through the living and abiding word of God’ (1 Pe 1:23). ‘Of his own will he brought us forth by the word of truth, that we should be a kind of firstfruits of his creatures’ (Jas 1:18). In John 6 Jesus says, ‘No one can come to me unless the Father who sent me draws him’ (v. 44). Humans do not effect this new birth. ‘It is the Spirit who gives life; the flesh is no help at all,’ yet he immediately adds, ‘The words that I have spoken to you are spirit and life’ (v. 63, emphasis added).
“If this is the case, why do we need an immediately infused habitus to intervene between these mediated events? Does such an adaptation of this medieval category save us from synergism only to open the door again to a dualism between God’s person and Word? According to the above-cited passages, the Spirit implants the seed of his Word, not a principle or habit distinct from that Word. At no point in the ordo salutis , then, is there an infusion of a silent principle rather than a vocal, lively, and active speech. In attributing all efficacy to the Spirit’s power, Scripture nevertheless represents this as occurring through the Word of God that is ‘at work’ in its recipients (1 Th 2:13; cf. 1 Co 2:4–5; 2 Co 4:13; Eph 1:17; Gal 3:2; 1 Th 1:4; Tit 3:4)—specifically, that message of the gospel, which is ‘the power of God for salvation’ (Ro 1:16; 10:17; 1 Th 1:5).
“Therefore, the external call includes the locutionary act of the Father’s speaking and the Son as the illocutionary content. The internal call (effectual calling), synonymous with regeneration, occurs through the Spirit’s perlocutionary effect. As in all of God’s works, the Spirit brings to fruition the goal of divine communication. The Father objectively reveals the Son, and the Spirit inwardly illumines the understanding to behold the glory of God in the face of Christ (2 Co 4:6; cf. Jn 1:5; 3:5; 17:3; 1 Co 2:14), liberating the will not only to assent to the truth but to trust in Christ (Eze 36:26; Jer 32:39–40; Heb 8:10; Eph 2:1–9). Regeneration or effectual calling is something that happens to those who do not have the moral capacity to convert themselves, yet it not only happens to them; it happens within them, winning their consent. The God who says, ‘Let there be. . . . And there was . . . ’ also says, ‘Let the earth bring forth . . . ’ Because the Word of God is not mere information or exhortation but the ‘living and active’ energies of the triune God, it is far more than a wooing, luring, persuasive influence that might fail to achieve the mission on which it was sent. In both instances, it is the work of the Father, in the Son, by the Spirit.”
In a footnote, Horton quotes this from Witsius to illustrate the earlier viewpoint: “Regeneration is that supernatural act of God whereby a new and divine life is infused into the elect person, spiritually dead, and that from incorruptible seed of the word of God, made fruitful by the infinite power of the Spirit ” (emphasis added).
I would only add that the view that regeneration is an “immediately infused habitus ” looks like a throwback to medieval soteriological categories that the Reformation was trying to escape.