To anybody who is even slightly familiar with your voice, other people's words sound alien coming out of you. They don't match your rhythm, your melody, or any of the other identifiers of your voice. This can be a real problem when you incorporate other people's words wholesale. Your text ends up sounding like a patchwork of voice.
If you are going to use another person's ideas, do not also use their words. Break down to a conceptual level what they're saying, and then explain that concept in your own words. When you do that, it becomes a lot easier to blend it in with the rest of your writing. It may sound a little strange if the subject matter is noticeably uncommon, but it will at least sound like your thoughts if you can say them in your voice.