From a priest:
Do you know when exactly the formula for giving Communion to the faithful was changed?
In the Ordinary Form the minister says “Corpus Christi… The Body of Christ” and the communicant responds “Amen”.
In the Extraordinary Form the minister says something closer to what the priest says before his own Communion “Corpus Domini nostri Jesu Christi custodiat animam tuam in vitam aeternam. Amen.” The minister says “Amen”, not the communicant.
My understanding is that Paul VI in 1964 said said that the formula should be shortened. Inter oecumenici 48, i. There was a follow up decree Quo actuosius, beginning:
“In order that the people may more actively and beneficially take part in the sacrifice of the Mass and profess their faith in the eucharistic mystery in the very act of receiving communion, numerous requests have been submitted to Pope Paul VI for a more appropriate formulary for the distribution of communion.” (BTW… I don’t think the claimed motive worked.)
“Corpus Christi” is what was said in the Ambrosian Rite (Paul VI had been Archbishop of Milan, of course.) and the shorter form is attested to both Sts. Ambrose and Augustine. The shortened form was therefore picked up for the Novus Ordo.
Ambrose (probably) wrote in his work to the newly baptized (therefore Augustine would have something like this from Ambrose own lips) De sacramentis 4.25:
“So you say not indifferently ‘Amen’, already confessing in spirit that you receive the body of Christ. Therefore, when you ask, the priest says to you: ‘the body of Christ’, and you say: ‘Amen’, that is, ‘truly’. What the tongue confesses let the affection hold. That you may know, moreover: ‘This is a sacrament, whose figure went on before’.”
Augustine said in s. 272, preached perhaps in 408 on Pentecost, to the infantes (newly baptized) about the fact that we see one thing (bread) but we receive another (Christ).
“What you hear, you see, is ‘The Body of Christ,’ and you answer, ‘Amen’. So be a member of the body of Christ, in order to make that ‘Amen’ true.”
In any event, despite the Patristic pedigree, in this time of weak Catholic identity, we should by means of the “gravitational pull” exerted by the Extraordinary Form, return in the Novus Ordo to the older form of distribution, perhaps beginning on weekdays. Right now, that would be contra legem.