Basic: How can I stop centering a text?

1. Note that the approach of {\centering Text} of the accepted answer will not work because there are not a blank line (=\par) before of the closing brace. The same apply to \begingroup\centering ... \endgroup

2. Note also that the center environment add a noticeable vertical spacing before and after the caption (that could be or not what you want).

3. For one line caption without additional space is enough a \hfil after the equation.

4. If you left a blank line between a display math mode and \hfil, be aware that the text is centered taking into account the paragraph indentation.

5. Alternatively to \hfil you can use also a \makebox wide as to the line width, since its content is centered by default. Unnecessarily complicated to obtain the same, unless you want a wide \fbox around the caption or so.

MWE (blank lines are converted to \par to remark where they are.):

\documentclass{article}
\begin{document}
\[ a^2+b^2=c^2 \]
\hfil This is the Pythagorean Theorem.\par           % works 
\hfil This is the Pythagorean Theorem.\par           % works if \parindent is 0pt
\begin{center}
This is the Pythagorean Theorem.                     % note the vertical spacing 
\end{center}
{\centering This is the Pythagorean Theorem.}\par    % oooooopps !
{\centering This is the Pythagorean Theorem.\par }   % works
\noindent\makebox[\textwidth]{This is the Pythagorean Theorem.}\par % works too
\end{document}

Edit:

By the way, to use formula with captions I will make a new type of float or true captions without floats, MWE:

\documentclass{article}
\usepackage{amsmath,lipsum}
\usepackage{float}
\floatstyle{boxed}
\floatname{eq}{Equation}
\newfloat{eq}{H}{eqn}
\usepackage{caption}
\captionsetup[eq]{skip=20pt,position=top}
\begin{document}
\lipsum[2]

\begin{eq}
\[ a^2+b^2=c^2 \]
\caption{This is the Pythagorean Theorem.}
\end{eq}

\begin{eq}
\[\mathbf{F} = \frac{d \mathbf{p}}{dt} = \frac{d (m \mathbf{v})}{dt} =  m \frac{d \mathbf{v}}{dt} = m\mathbf{a}\]
\caption{Newton's second law of motion}
\end{eq}

\lipsum*[13] Or maybe mora compact and simple fake floats: 

\[ a^2+b^2=c^2 \] 
\captionof{eq}{This is the Pythagorean Theorem.}

\[\mathbf{F} = \frac{d \mathbf{p}}{dt} = \frac{d (m \mathbf{v})}{dt} =  m \frac{d \mathbf{v}}{dt} = m\mathbf{a}\]
\captionof{eq}{Newton's second law of motion}

\lipsum*[2] 
\end{document}

Although really I prefer the classic numeration at right of formulas, without captions. ;)

Another possibility: use the gather environment (a multiline, centred, maths environment), and put the ‘caption’ on the second line:

\documentclass{article}

\usepackage{amsmath, amssymb}

 \begin{document}

\begin{gather}
   a\not\equiv 0\mod p \implies a^{p-1}\equiv 1\mod p \\
  \text{\small This is known as \emph{lil’Fermat}.}\notag
\end{gather}

 \end{document}