\documentclass{article}
\usepackage{amsthm}
\theoremstyle{theorem}
\newtheorem{theorem}{theorem}
\begin{document}

\begin{theorem}
lllllllllllllllllllllllllllllllllllllllll 
\\
\emph{llllllllllllllllllll llllllllllllllllllll }
\end{theorem}
\end{document}