Know what I'm gonna do? In keeping with the spirit of my OP, I'm only going to say nice things about the video and David Platt.
...Which isn't hard. So I was originally going to make notes on places where he said something inaccurate. While there are a few little things I'd correct, overall his summary of LDS doctrine isn't bad. I'm not gonna go into nitpick mode over it. As an overview of LDS theology as seen by a Protestant, it was one of the more accurate I've heard.
So one thing I'd disagree with is his referring to the LDS Church as a cult, but to be fair he acknowledges that a Mormon wouldn't agree with that (Yeah, it's obvious but it's nice that he points it out.) and would say the opposite is true, that the LDS perspective is that it restores the original Gospel.
Yes, he's right, but I'd just like to clarify that it does NOT mean we label other denominations as cults. I mean, some might but I've never heard a Mormon say that. It just isn't constructive. I have, once or twice, heard a Mormon openly attack Catholicism. I didn't like that very much though, for the same reason. The guy didn't accuse us of that or anything, I just felt there was value in pointing it out.
He mentions that the majority of members of LDS don't know all the theological details, but he also says that's also true of Christians in general. I think that's fair, in the sense that most people in general aren't interested in the details and that's reflected in these numbers across denominations, assuming they're accurate (And I don't have reason to question them at this point.)
It's interesting that he points out there isn't a single authoritative source that represents all Christians' point of view. He's obviously right about that. To me that's a problem but, to be fair, I also don't know what the solution is, other than for the Savior to come and establish His single Church with authority granted by Him to represent His true Gospel...
So from there he talks about wanting to fairly and accurately represent what Mormons believe. I definitely appreciate that, and I think he made a sincere effort to do that. I've often seen people write books, give speeches, etc. supposedly about what we believe but they're often highly inaccurate. I honestly got excited to hear what he had to say.
He frequently refers to Protestant Christianity as "foundational" I think it's important to note that because it frames the point of view from which this talk is given. So the skepticism is perfectly understandable given the differences between Protestants and LDS. And yeah, there are certainly a lot of them but one thing becomes clear: If the Protestant perspective is correct, then the LDS Church must be false. On the other hand, if even one single claim made by Mormons is true, then they're ALL true, because they're inextricably bound, and thus the LDS Church is exactly what it claims to be.
Here's what I mean by that. Take any component of LDS theology and see if it's true. If it is, then all the other elements must be as well. I'll give an example. If the Book of Mormon is truly the Word of God just like the Bible, then Joseph Smith did indeed translate it by the power of God. If that's true, then he was indeed a prophet. If that's true, then the LDS Church is led by a prophet even today. If any single element in that chain is false, then it all collapses.
I like how he acknowledges the value Mormons place on good and moral things, which is the fruits by which you know us (Christians) as discussed in a thread on this forum not too long ago. It's a standard I'd be pleased to be judged by, because it's Christ's standard.
I liked that more than once he points out that there are 70,000 LDS missionaries out there in the world. I also liked that he used that fact to encourage more missionaries from Protestant churches to get out there. The world needs Jesus now more than ever. One thing I'd point out though, and this is super important: LDS Missionaries don't go out there expecting you to take their word for it. Any LDS missionary you talk to will tell you the same thing: Ask of God if the things they say is true. (James 1:5, which the guy mentioned, is huge here.) Now, when I was first talking to the missionaries, they told me that too, and it really impressed me because that's consistent with what I would expect true representatives of God to say. "Don't take my word for it, ask God for yourself." I have never once heard that from anybody else.
I also really liked that he encouraged the audience to pray for us, as well as to show love and kindness. That's how we should ALWAYS act toward each other, all of us, all the time. It's great advice. Yeah, I get that he meant that as an approach to try to convert us, but I'd still much rather be shown kindness with that motive than criticism or confrontation. Plus, I love that he asked them to pray for us. I've been prayed for at various times for various reasons by Catholics, Protestants, Mormons, and even a Muslim or two. That's so awesome. How great an expression of love it is for someone to call upon God, the CREATOR OF THE WORLD on one's behalf? That really is amazing.
So yeah, the talk was meant to elaborate on why he thinks the LDS Church is a cult. That sort of thing used to bother me a LOT, because I felt like it was wrong for someone to presume to judge what's in my heart where only Jesus Christ can do that. Now it doesn't really bother me anymore, and for the same reason. The only one who I care about seeing me as a follower of Jesus Christ is... Jesus Christ. Everybody else's opinion is their opinion, which they're certainly entitled to.
One thing I still don't get... why is it called "Secret Church?"