Using package amsthm, the desired theorem style achieved,
\documentclass{book}
\usepackage{amsthm}
\newtheoremstyle{mystyle}{}{}{}{}{\bf}{}{\newline}{}
\theoremstyle{mystyle}
\newtheorem*{mythm}{Theorem}
\begin{document}
\begin{mythm}
This is a normal body text.
\end{mythm}
\end{document}
But itemizing the body makes trouble,
\documentclass{book}
\usepackage{amsthm}
\newtheoremstyle{mystyle}{}{}{}{}{\bf}{}{\newline}{}
\theoremstyle{mystyle}
\newtheorem*{mythm}{Theorem}
\begin{document}
\begin{mythm}
\begin{itemize}
\item This an itemized body text.
\item This an itemized body text.
\end{itemize}
\end{mythm}
\end{document}
As the last output shows, first item has jumped to the head line.
How can I bring it back to the body?




\leavevmodebefore\begin{itemize}– Apr 30 '16 at 13:05\leavevmodeis exactly what that answer recommends, as christian has repeated. items are not aligned with the head line, but treated as a "usual" list. i'll add an example to the cited answer to make this clear. – barbara beeton Apr 30 '16 at 13:42leavevmodeis not enough, as there will be a spurious vertical space. – Bernard Apr 30 '16 at 15:00