Symfony3: how to invoke a custom twig function in a twig template?

Question

I have a main twig template same as below:

# main.html.twig

{% block main %}
{{ layout|raw }}
{% endblock %}

in variable of layout, where has a custom twig function: customFunction that is a html code:

title

there has some text

But now, when I render the main.html.twig:

{{ customFunction(4) }}

not is:

<h3>title</h3><p>there has some text</p>

how to solve it?