How does marketing affect customers loyalty?

It's really important.  A bad marketing campaign can alienate the existing customer base and severely damage sales.  A really bad advertising campaign can result in formerly loyal customers, now offended, demonstrating outside offices and making threats of violence against the staff.

Marketing involves creating awareness, communicating value & satisfying customer needs more than competitors on a sustainable basis. If a company effectively & efficiently addresses the above objectives, its most likely to achieve customer loyalty BUT if it fails to do so, then its most likely to miss out on customer loyalty.