The example given by the OP is acceptable. OTOH, what happened today (again!) at my local parish is not acceptable: Lay preaching! Presiding priest says two sentences; then invites lay person to preach. I will say that unlike all the previous
four such incidents within the last 12 months (!), this one wasn’t quite as difficult to listen to as the previous four, in that the speaker’s content was a tiny bit less dry, but ever so tinily. Still, though --like the others-- it was way too academic. A homily is not an academic talk, no matter how learned the content, how educated the priest. There’s an art to it, and generally it is not read, at least not as literally as one reads an academic paper.
Not a deacon, not a seminarian, not a missionary, not a visiting priest. Not an appeal for a cause. Merely a lay person giving a talk about the Trinity, on Trinity Sunday – replacing the priest’s homily.
Despite my frequent complaints here, I’m not a stubborn purist. I think the laity has a lot of ideas to give priests, if/when priests are willing to solicit and incorporate those ideas. I actually think that’s a good use of lay wisdom. (A good homily should have a lot of wisdom in it!) In conversations that come up with priests and pastors, regarding insights, for example – theological insights, scriptural insights – I think it’s perfectly acceptable for a lay person to say to the pastor, “I’d love to hear your opinion on ______ and perhaps include your interpretation of that in a homily.” Or if the pastor appreciates a remark shared by a parishioner, for the pastor to ask permission to make oblique or direct reference to that, anonymously, in a homily. I see nothing wrong with that. That is very different from usurping the priest’s role. Again, to me this is not about canonical purity: it’s about the flow of the liturgy proceeding from a single leader, head, representative of the congregation. The Mass is not a democratic event with egalitarian participation and indistinguishable roles.
End of rant.
(I love Trinity Sunday!)