Hello Feng-
Thanks for reaching out, and thanks for using the Post and Page Builder for WordPress.
It looks like your theme has a relatively small container for the main page content, only 67% of an already-contained page width:
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I’m not very familiar with your theme, Responsive Church Theme, but if it offers an Additional CSS setting in your Customizer, you can add this CSS to make that container full-width:
main#content {
width: 100%;
}