documentclass{article}

usepackage{enumitem}
newtheorem{theorem}{Theorem}[section]

begin{document}

setcounter{section}{2}% Just for this example
section{A section}

setcounter{theorem}{1}% Just for this example
begin{theorem}
  The set has the following parameters:
  begin{enumerate}[label={(thetheorem.arabic*)},leftmargin=*]
    item Non-empty and
    item Compact.
  end{enumerate}
end{theorem}

end{document}