When properly nuanced, and correctly explained and taught---I should now say I believe in "presumptive regeneration."
The key for me is the separate theological doctrines of regeneration and conversion, and the ordo salutis. Regeneration has to do with the granting of a certain capacity to a person, a change in nature. Conversion has to do with the exercise of that faculty as designed. Where the former is effected, the latter will certainly be also, as G.I. Williamson puts it: "regeneration is inseperable from its effects." While that may be often practically simultaneously, which leads to the common assumption that these two--capacity and exercise, cause/effect--must be found together at all times, or neither is there. But that is not necessarily the case. A baby in utero has the capacity to see, but unborn in the absence of light, sees nothing. Likewise a person with his eyes closed asleep, sees nothing--but when awakened does see, having had the capacity to see even while unseeing.
Regeneration is tied, in the ordinary means of grace, to the preaching of the Word. So, it is effectual calling in the ministry of the Word that ordinarily effects regeneration, and regeneration that makes it possible to "hear" the Spirit doing the calling. In this sense, the two are so closely identified that putting the one ahead of the other is impossible. The action of the God the Spirit (a powerful calling) precedes the activity of the person (hearing), because he is the initator, the creator, the living One; and the man is first of all dead and passive. But the hearing must be "active" so as to receive the call! So the call does it all--calls and grants hearing (regenerates) in one act.
But it is the spirit of a man (not his organism) that "hears" the Voice of the Spirit. So, there must ordinarily be a divine coordination between the spoken Word, and the Spirit-given Word. Hence the necessary concepts of the means of grace and a mediated Word by the throat of a minister.
When I preach to the congregation, I am in fact presuming certain things: first, that those in my hearing have the capacity to receive spiritual truth, i.e. are regenerated. This is both the teaching aspect of my ministry, as well as a portion of the converting aspect. Some of these may have recently been given the capacity to see spiritually (new life, on another occasion of the Word), and the conversion being "in process," unto the full exercise of faith and repentance.
I also presume that some of my audience may, to begin with, not be regenerated, but I believe that by the power of the gospel and the Spirit, he may use that Word and coordinate his inward call so that faith might come by the hearing. Regeneration has the seed of all the rest of the divine work already present in it (conversion, justification, adoption, sanctification, glorification). And by the Word and Spirit, the remainer comes about.
So where does "presumptive regeneration" fit into this? Just here: I "presume" that the children in this church are being called, converted, and taught, all three in the ministry of the Word. The only way the hearing of the Word will do them any spiritual good is if the Holy Spirit is savingly active (toto ordo) in its presentation. But I don't know where they are, spiritually--no more than I know the state of any of the adults present. Some of them may have been regenerated in the womb. Many, or even most in some churches, have been under Spirit-empowered preaching and spiritual instruction from parents and teachers thousands of times before they were 2 years old!
Every one of these teachers should have "presumed" that these children were capable of receiving rudimentary spiritual instruction, or else that by the very act of spiritual instruction, God would impart spiritual life, and so would be regenerated by the time they finished talking.
Here's yet another application: what good is a "truncated" gospel? I know of churches where all the spiritual instruction the youngest children receive, or Sunday School up through... whenever, is nothing but "have you received Jesus? have you received Jesus?" Or, the "Bible story" is taught, some "facts" and perhaps a moralistic life-lesson, but with no thought that the youth are really capable of receiving spiritual good until they've "received Jesus."
Until then, the goal is nothing but rearing compliant, respectful pagans. Hence, every class concludes with a "bow your head, close your eyes, raise your hand if you want to receive Jesus in your heart today." And come to think of it, isn't this what happens a lot in the regular service as well? Spiritual instruction is something that happens on your own, or in small groups, or post-conversion Suday School classes. There is no unifed message! The Spirit's gospel includes the "what wilt thou have me to do?" part as well.
As I see it, the difference in preaching the gospel properly is that it is never separated from the imperatives. And this holds in teaching down to the youngest level. There's no waiting for a profession, before assuming that one can speak cogently to the spirit of a child. We've no idea when and where and how far the Spirit has moved on that little heart thus far.
"Oh, but what if that child is not one of the elect?" And? We don't ask that question about adults, or adjust our preaching accordingly. The non-elect are like ground that drinks in the rain, only to produce thorns and thistles. This is apostasy--the receiving of spiritual things, without finally sensibly appreciating them spiritually, because one is incapable of receiving them, being a natural man unto the end. But none of that is in our purview.
I think I have spiritual duty to presume the best, that those in my hearing who belong to the church are capable of receiving the Spirit of Truth. While at the same time acknowledging that this may not in fact be the case, and that there be those elect on whom the Spirit will yet be a long time pursuing, before beginning a saving work in them, as well as non-elect persons present. And my preaching unto them will be for the storing up of wrath against the day of wrath.
However, in the final analysis, I still do not like the terminology "presumptive". I feel as though it says just a bit too much; it goes over the line. Erroneous presumption is really "presumptive conversion" or "presumptive Christianity apart from the means of grace." True "presumptive regeneration" insists on seeking and drawing out the evidence of that regeneration, and is never satisfied to "presume it all" or "presume upon God's converting grace."
Tell me, would any decent preacher be content to ignore the sins of his people, because "they're saved, that's all forgiven"? Perish the thought! May God have mercy on that minister, because I've already judged him worthless.
If you presume regeneration, you better practice church discipline too, and "turn that person over to Satan," hoping even in excommunication for his recovery and reinstatement when he finally turns and repents. "Love hopes all things." If you presume regeneration, you better use all the means of grace, and all the other divine appointments unto blessed ends. Only then may you with a clean conscience leave the outcome in the hands of a merciful and all-good God. And you shoudl still pray for God to forgive your less-than-perfect use of those means. And finally pray that God would give you peace even if the person you once presumed upon was not one of God's after all.
Since I now confess I presume regeneration, I'd better do it properly.