documentclass[a4paper, 12pt]{report}

usepackage[ngerman]{babel}
usepackage{geometry}
usepackage{indentfirst}
usepackage{ntheorem}
usepackage{lipsum}

geometry{outer=20mm, inner=25mm, top=20mm, bottom=30mm}

theorembodyfont{normalfont}
theoremseparator{:}

newtheorem{hyp}{Hypothese}

begin{document}
chapter{}
lipsum
begin{hyp}
Here should be Hypothesis number one with first line indented and the remaining text normally over the whole page.
end{hyp}

begin{hyp}
Here is another Hypothesis with an intended first line and remaining text like in the remaining thesis.
end{hyp}
end{document}