In light of the fact that Donatism is heresy, why do most churches require a priest to bless the elements? As the church was emerging from its persecutions under Diocletian and into legitimacy under Constantine, there arose a schismatic church named the Donatists.  Now, as one might imagine in a time of persecution, there were priests who, when given a choice between denying Jesus and being thrown to the lions, did in fact, deny their Lord.  A question then arose as to whether or not the Eucharist that these priests celebrated was legitimate - the Donatists said 'No,' but the official orthodox position of the church (especially as articulated by Augustine), was that 'Yes,' in fact the Eucharist was valid because, &quot;the efficacy of sacrements is not affected by the worthiness of celebrant.&quot;
That idea - that the Eucharist is not affected by the worthiness of the celebrant - seems to be quite a firecracker if taken to a seemingly natural conclusion.  Namely, if the efficacy of the sacrement is strictly ex opere operato (meaning from the work performed) and not ex opere operantis (from the worker who performs it), why then is a priest required to bless the elements, in most traditions?
So, some disclaimers:

Disclaimer #1:  Yes, some Radical Anabaptists like John Smythe had no problem celebrating the Eucharist even without ordination, but it is a fairly fringe case.  Even many traditional mainline Baptist churches require that the elements be blessed by the minister, and without that, it is not considered communion. Certainly within the Catholic, Anglican, Presbyterian, and even I suspect the Methodist churches, if the Youth Minister decided to bless the elements, he'd get a minor toungue-lashing for the offence.

Disclaimer #2: Yes, there are some churches that don't believe Donatism is a heresy.  They are fringe cases - the vast majority of the church holding that Donatism is, in fact, heresy.

Disclaimer #3: This question touches on why Catholics believe a priest is required.  In an otherwise excellent answer, the key part of the question that I am interested in (Why can't laity bless the elements), I read:

In terms of why the laity cannot yield the effect of transubstantiation, I don't have an extensive enough knowledge to pin that question down precisely.


Disclaimer #4: I am fully aware of the emergency situation where any priest - defrocked, heretic, or otherwise, is obliged to administer the sacrement of absolution.  A good answer may extend this case, but should reference appropriate sources.


So, with all that said, Why can't the laity bless the elements in most churches?  Is there a biblical case to be made, or is it just tradition.  If the answer is 'tradition,' is it canon law? (Cites would be fabulous, and various denominational rules are acceptable!) Contratriwise, are there denominations (other than LDS - who I do believe would allow this!) that would say, for example, &quot;Sure, the father can serve communion to his wife and children&quot; ?
In short, why do we need priests (Catholic or Protestant!) to bless the Eucharist, if in fact, Christ is the one who is present in the elements to begin with?
As far as the Roman Catholic Church and the Church of England are concerned, it certainly is Canon Law.

900 §1. The minister who is able to confect the sacrament of the Eucharist in the person of Christ is a validly ordained priest alone.
907 In the eucharistic celebration deacons and lay persons are not permitted to offer prayers, especially the eucharistic prayer, or to perform actions which are proper to the celebrating priest.
[CLC]

B 12 Of the ministry of the Holy Communion

No person shall consecrate and administer the holy sacrament of the Lord's Supper unless he shall have been ordained priest by episcopal ordination in accordance with the provisions of Canon C 1.

[Canons of the Church of England]

That the Eucharist is not affected by the worthiness of the minister is also enshrined in a number of church documents (Article 26 in the Church of England, for example).
However, an unworthy minister is still a minister. The ontological change wrought by the Spirit at ordination is what makes a man able to effect the sacrament. Whether he is worthy or not is of no consequence. A layman has not undergone that ontological change, and so is unable to celebrate the Eucharist. He can say the words and do all the right actions, but it is simply not effective.
The Catechism of the Catholic Church addresses this:

1141 The celebrating assembly is the community of the baptized who, &quot;by regeneration and the anointing of the Holy Spirit, are consecrated to be a spiritual house and a holy priesthood, that . . . they may offer spiritual sacrifices.&quot;9 This &quot;common priesthood&quot; is that of Christ the sole priest, in which all his members participate:10

Mother Church earnestly desires that all the faithful should be led to that full, conscious, and active participation in liturgical celebrations which is demanded by the very nature of the liturgy, and to which the Christian people, &quot;a chosen race, a royal priesthood, a holy nation, a redeemed people,&quot; have a right and an obligation by reason of their Baptism.11

1142 But &quot;the members do not all have the same function.&quot;12 Certain members are called by God, in and through the Church, to a special service of the community. These servants are chosen and consecrated by the sacrament of Holy Orders, by which the Holy Spirit enables them to act in the person of Christ the head, for the service of all the members of the Church.13 The ordained minister is, as it were, an &quot;icon&quot; of Christ the priest. Since it is in the Eucharist that the sacrament of the Church is made fully visible, it is in his presiding at the Eucharist that the bishop's ministry is most evident, as well as, in communion with him, the ministry of priests and deacons.
9 Lumen Gentium 10; cf. ⇒ 1 Pet 2:4-5.
10 Cf. LG 10; 34; Presbyteriorum ordinis 2.
11 Sacrosanctum Consilium 14; Cf. ⇒ 1 Pet 2:9; ⇒ 2:4-5.
12 ⇒ Rom 12:4.
13 Cf. PO 2; 15.