documentclass{article}
usepackage{amsthm, amsmath}
usepackage{enumitem}


newtheoremstyle{MyTheorem}% name of the style to be used
  {}% measure of space to leave above the theorem. E.g.: 3pt
  {}% measure of space to leave below the theorem. E.g.: 3pt
  {}% name of font to use in the body of the theorem
  {}% measure of space to indent
  {}% name of head font
  {}% punctuation between head and body
  {}% space after theorem head; " " = normal interword space
  {}% Manually specify head


theoremstyle{MyTheorem}
newtheorem*{theorem}{Theorem}


begin{document}

begin{theorem}
Any $ell in L$ is a root of an irreducible polynomial $p in F[x]$ which splits in $L$ into distinct linear factors.
end{theorem}

end{document}