\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{amsmath}
\usepackage{xypic}

\begin{document}
\begin{equation*}
  \xymatrix@R+2em@C+2em{
  A \ar[r]^-f \ar[d]_-\varphi & B \ar[d]^-\psi \\
  C \ar[r]_-g & D
  }
 \end{equation*}
\end{document}
