\documentclass[landscape,twocolumn,10pt,a4paper]{article}

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

\usepackage{dogmatics}

\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}

\usepackage{marginnote}
\reversemarginpar

% Smaller font in marginpars
\makeatletter
\long\def\@ympar#1{%
  \@savemarbox\@marbox{\scriptsize #1}%
  \global\setbox\@currbox\copy\@marbox
  \@xympar}
\makeatother

% Indexing
\usepackage{makeidx}
\makeindex

% Show index entries in the margin notes; see http://tex.stackexchange.com/q/236373/23992
\usepackage{xparse}
\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}

\printindex

\end{document}