Let’s get a couple things out of the way real quick. Copying someone else’s essay online and turning it in for your Freshman Comp class assignment is bad. Don’t do it. The point of the assignment is for the professor to see how you write and think, and you’re cheating when you pretend someone else’s stuff is yours. Taking swathes of someone else’s research and presenting it in your book without attribution, as if it was yours, is wrong. Thou shalt not. Likewise, watching a John Piper sermon on Youtube and then delivering that same sermon to your congregation, pretending that you wrote it, telling his story of what happened in the grocery store line as if it happened to you — that’s wrong, mmkay? (I’ve written about this last case before, and the problem is much bigger than plagiarism, it’s dereliction of pastoral duty.) I hope these disclaimers go without saying, but since I’m going to redraw some lines here, I guess I’d better say them.
Those things said, I argued last week that our contemporary take on plagiarism is a historical and cultural oddity founded on highly questionable presuppositions. Even here in the West, we didn’t think that way about authorship until very recently. The older model, the one that obtained throughout the ancient world and right on through Christendom, made very free use of source material, and at the same time made very free modification and adaptation of that source material. Everything was presumed to be a derivative work; what kind of idiot would try to compose anything of significance totally on his own?
Which is to say, they had an ethos of apprenticeship. You mimicked the best. You made modifications as your own vision and situation called for it. That’s how they did everything.
That approach to composition is largely dead, but it survives in effective preaching. A sermon is not a novel. When I get up to speak on (say) Ephesians 1:3-14, there is no expectation that I am going to say something unique in the history of exegesis and theology. In fact, very much the opposite. The goal is to say things that are true, and nourishing for the people God has given me to serve. In service of that end, I am able to make very free use of source material, and at the same time make extensive modifications to it to make it suit my setting and situation, If the “hungry sheep look up, and are not fed,” then I didn’t do my job. But if they were fed, then I did do my job, and that’s all that matters.
There is a type of preacher that will liberally festoon his sermon with verbal footnotes, lest he be accused of plagiarizing something. I understand the motivation, but that kind of name-dropping is just bad practice. The emphasis should be on the Word, not on how much homework you did and who you read when you were doing it. Don’t give your people a list of authors; give them what the text says and what to do about it.
Now of course, if someone asks you where you got a particular point, feel free to point them to the author and work you got it from. You shouldn’t conceal your sources. But you don’t need to flaunt them either. Flaunt the Scriptures.
Full Contact Christianity 
We should be glad that people copied and preached our sermons with acknowledging us, as long as Christ is glorified and his Gospel is proclaimed! After all, it’s God’s word that’s being proclaimed, not any man’s!
*without acknowledging us