\newcommand{\arrow}[2]{\path [-{Latex[scale=1]}] (#1) edge (#2);}

\begin{tikzpicture}

  \begin{scope}[every node/.style={shape=circle, fill=white, draw, inner sep=2pt}]
    \node (A) at (0,2.0) {$A$};
\node (B) at (-2.0,0.0) {$B$};
\node (C) at (-0.0,-2.0) {$C$};
\node (D) at (2.0,-0.0) {$D$};
  \end{scope}

  \begin{scope}[every draw/.style={}]
    \draw (A) -- (B);
\draw (A) -- (C);
\draw (B) -- (D);
\draw (C) -- (D);
  \end{scope}

\end{tikzpicture}