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, "the efficacy of sacrements is not affected by the worthiness of celebrant."
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, "Sure, the father can serve communion to his wife and children" ?
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?