\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}