Italic or font change for a whole section

Question

I would like to change the appearance of a whole section in a text with math content. I have for instance a half page with several equations in it which I would like to be differentiated from the rest of the text by being written in italic or in another font. When I use the command \textit` for instance the compiler tells me:

Paragraph ended before text@command was complete}

Too many }'s }

Is there a way to do this?

EDIT: I am writing my master thesis using:

\documentclass[a4paper,10pt]{report}

\newcommand{\separator}{\begin{center}
\vspace{5mm}
 $* \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad * \qquad *$
\vspace{5mm}
\end{center}}

\separator

Text and equations ...

\separator

What I would like is additionally to have everything between separator to be written in a different font or textstyle. I tried putting a \textit{...} wrapped all around, but it gave the errors mentioned above.

Answer 1

\textit is command that takes an argument, it also is short, meaning you cannot have multiple paragraphs in there. A proper way would be to use \itshape and use it in a group, best to define your own environment.

There are many ways to make this material stand out and recognized not to be part of the usual text. Below, i decided to use lines as well as a shift into the margin to make a clear distinction.

\listfiles
\documentclass{article}
\usepackage{blindtext}
\usepackage{scrextend}
\usepackage{mathtools}[2015/06/17]
\newenvironment{suppmat}{\begin{addmargin}[1cm]{-1cm}\noindent\rule{\linewidth}{.3pt}\smallbreak\itshape}{%
        \smallbreak\noindent\rule{\linewidth}{.3pt}\end{addmargin}\medbreak}
\begin{document}
\blindtext
\begin{suppmat}
    \begin{equation}
        p=mv
    \end{equation}
    Muss man wissen!
\end{suppmat}
\blindtext
\begin{suppmat}
    This could be some explaining text, we need it to show
    that this stuff can break. \blindtext
    \begin{align}
        E &=mc^2
        \shortintertext{this is short text}
        a^2 + b^2 &= c^2
    \end{align}
\blindtext
\end{suppmat}
\blindtext
\end{document}

Answer 2

As Johannes_B was correctly pointing out, \textit can only take an argument (e.g. a few written lines) and cannot handle multiple paragraphs.

The simplest solution for me was to use itshape as follows:

{\itshape
Write here everything that you want in italics. 

It can handle different paragraphs and equations (although the equations will not be in italics)
}

Another option is to use \begingroup \itshape text here will be italics \endgroup or \begin{itshape} text here will be in italics \end{itshape} (Thanks @ChristianHupfer for the suggestion, and mentioning that the last solution works but it's not elegant)