Iustin mentioned in his interview that he did the sermons twice daily and thought he was doing a bad job of them, because he had no time to prepare them. So De Luna offered to take them over at least while he's here, not because he's any better suited to it - he gives sermons to the priests in training and in Vigil, which is a very different skillset - but because perhaps his errors will be in different directions and useful to Iustin. He has been to the best of his abilities sticking to the very basics, but it can be easy for someone who spends most of their free time getting into complicated theology debates to assume that an ignorant ordinary person is familiar with two or three of the basic philosophical distinctions the Acts emphasizes.
He's also trying not to be political, so he has avoided giving the sermons he'd very much like to on Iomedae's relationship to Taldor, or lately on censorship.
His Sunday sermon is a reading from a part of Acts that's always popular, the part where Iomedae holds a fortress against overwhelming odds with badly injured men until sunrise in the Second Battle of Encarthan, and then the most basic explanation of it he can possibly give: despair is a sin, and the root of many more dangerous sins, and even in the face of overwhelming odds one can and must choose to keep trying to do the best possible thing. But also, despair is usually an error: there are very few things that are as intractable as they look. A persistent theme in Acts is people telling Iomedae that there's no way to do the thing that needs doing, or no way to do it without great evils. No way to hold this fortress, no way to take that one, no way to conduct interrogations without torture, no way to keep discipline without brutality, no way to have soldiers without whores. Despair, all of it; false, all of it.
And then there are all of the necessary caveats: the fact there is some way to do better doesn't mean that any random way you thought of will be better, the fact that no position is hopeless does not mean it's not an important responsibility to get into the position from which winning is likeliest; that despair is a sin does not make optimism a virtue, because to willfully deceive yourself into thinking a situation better than it is is also foolish. Iomedae, speaking to the dying men, does not claim to them that they will not die; speaking to the living ones she does not claim to foresee a victory that she was at the time not sure of. She was instead steadfast in her conviction that they had a little more in them, a little more courage, a little more stubbornness, a little more hope, and that the dawn was after all approaching, and their fate still in their hands.