\documentclass[landscape,twocolumn,9pt,a4paper]{article}

\title{An Introduction to Christian Theology\\\normalsize{Summary of the book by Plantinga, Thompson and Lundberg}}
\author{Camil Staps}

\usepackage{dogmatics}
\showquestionsfalse

\usepackage[top=2cm, bottom=3cm, inner=3.6cm, outer=3.6cm, heightrounded, marginparwidth=2cm, marginparsep=0.6cm]{geometry}
\usepackage[hidelinks]{hyperref}
\usepackage[english]{babel}
\usepackage{multicol}
\usepackage{array}

\makeatletter
% Align marginpars to inside; doesn't work yet
\let\oldmarginpar\marginpar
\renewcommand*{\marginpar}[1]{\oldmarginpar{\color{black}\if@firstcolumn\raggedleft\fi#1}}
%\renewcommand*{\marginpar}[1]{\oldmarginpar{\color{black}\raggedright#1}}
% Smaller font in marginpars
\long\def\@ympar#1{%
  \@savemarbox\@marbox{\scriptsize #1}%
  \global\setbox\@currbox\copy\@marbox
  \@xympar}
\makeatother

% Indexing
\usepackage{makeidx}
\usepackage{multicol}
\makeindex
% Index on same page; http://tex.stackexchange.com/a/23873/23992
% But in next column; http://tex.stackexchange.com/a/27481/23992
\makeatletter
\renewenvironment{theindex}
  {\vfill\eject\section*{\indexname}%
   \@mkboth{\MakeUppercase\indexname}%
           {\MakeUppercase\indexname}%
   \thispagestyle{plain}\parindent\z@
   \parskip\z@ \@plus .3\p@\relax
   \columnseprule \z@
   \columnsep 35\p@
   \let\item\@idxitem}
  {}
\makeatother

% Show index entries in the margin notes; see http://tex.stackexchange.com/q/236373/23992
\usepackage{xparse}
\renewcommand\qindex[1]{\oldindex{#1}}
\let\oldindex\index
\renewcommand{\index}[1]{%
  \oldindex{#1}%
  \marginpar{\bfseries\splitentry{#1}}%
}
\NewDocumentCommand{\splitentry}{ >{\SplitArgument{1}{!}} m }
 {%
  \splitentryEM#1%
 }
\NewDocumentCommand{\splitentryEM}{mm}
 {%
  \splitentryBAR{#1}%
  \IfValueT{#2}{, \splitentryBAR{#2}}%
 }
\NewDocumentCommand{\splitentryBAR}{ >{\SplitArgument{1}{|}} m }
 {%
  \dosplitentryBAR#1%
 }
\NewDocumentCommand{\dosplitentryBAR}{ >{\SplitArgument{1}{@}} m m }
 {%
  \splitentryAT#1%
 }
\NewDocumentCommand{\splitentryAT}{ m m }
 {
  \IfNoValueTF{#2}{#1}{#2}%
 }

% Title size
\usepackage{titlesec}
\titleformat*{\section}{\large\bfseries}

\begin{document}

\maketitle

\input{sum-chap-3.tex}
\input{sum-chap-4.tex}

\vfill\eject
\begin{multicols}{3}
    \printindex
\end{multicols}

\end{document}

