\centering
is a declaration and therefore its scope extends “forever”, but “forever” means “until the group where the declaration is issued ends”.
In your case \centering
is issued at the top level, so its scope extends up to the end of the document, unless countermanded by a similar declaration such as \raggedright
or \raggedleft
. There is no \uncentering
declaration, so grouping is necessary.
However, by rule of TeX, a paragraph is set with the values of some relevant parameters active at the point \par
is executed.
Thus, as in your previous question,
{\centering This text will not be centered}
And neither this one
will typeset two normally justified paragraphs, because the \par
implied by the blank line acts when the scope of \centering
has ended, restoring the paragraphing parameters to the values they had before the {
.
If you want centered text followed by justified text, you need to do two paragraphs:
{\centering This text will be centered\par}
\noindent And this text will start from the left margin.
However, good typography dictates that the centered part is separated by the justified one:
\begin{center}
This text will be centered
\end{center}
And this text will start from the left margin.
Examples
\documentclass{article}
\begin{document}
\section{Bad}
{\centering This text will not be centered}
And neither this one
\section{Ugly}
{\centering This text will be centered\par}
\noindent And this text will start from the left margin.
\section{Good}
\begin{center}
This text will be centered
\end{center}
And this text will start from the left margin.
\end{document}

\par
does not extinguish the effet of\centering
, which mainly sets\leftskip
and\rightskip
to infinitely stretchable glue so that text is centered. – user4686 Sep 20 '18 at 20:25\par
(or a blank line which creates a\par
token not written by user). Only then does it apply its algorithms to split the "horizontal" material into a paragraph composed of lines. – user4686 Sep 20 '18 at 21:15