According to you previous question A database for a Psalter and self-answer, you have something like
\newcommand{\PsalmI}{%
<text of Psalm 1>
}
\newcommand{\PsalmII}{%
<text of Psalm 2>
}
I removed the [1]
from the code you showed, because it's really wrong as you don't have an argument.
Now you want an interface like \Psalmus{22}
for typesetting Psalm 22. That's possible with standard (internal) commands of LaTeX:
\makeatletter
\newcommand{\Psalmus}[1]{\csname Psalm\@Roman{#1}\endcsname}
\makeatother
Let's try, with an abridged version.
\documentclass{article}
\usepackage[T1]{fontenc}
% this will probably be in a separate file that you \input
\newcommand{\PsalmI}{%
<text of Psalm 1>
}
\newcommand{\PsalmII}{%
<text of Psalm 2>
}
\newcommand{\PsalmXXII}{%
<text of Psalm 22>
}
%%%
\makeatletter
\newcommand{\Psalmus}[1]{\csname Psalm\@Roman{#1}\endcsname}
\makeatother
\begin{document}
\Psalmus{2}
\Psalmus{22}
\end{document}

On the other hand, I'd do it in a different way, without back and forth conversion into Roman numbers.
\documentclass{article}
\usepackage[T1]{fontenc}
% this will probably be in a separate file that you \input
\makeatletter
\newcommand{\definepsalm}[2]{\@namedef{psalm#1}{#2}}
\newcommand{\Psalmus}[1]{\@nameuse{psalm#1}}
\makeatother
\definepsalm{1}{%
<text of Psalm 1>
}
\definepsalm{2}{%
<text of Psalm 2>
}
\definepsalm{22}{%
<text of Psalm 22>
}
%%%
\begin{document}
\Psalmus{2}
\Psalmus{22}
\end{document}
\PsalmII
, just do it.