Your site was very most likely compromised and the malicious folks who did that included some base64 code in your functions.php file.
http://ottodestruct.com/decoder.php
You have addressed the 'symptom' but have not remedied the 'disease'.
If they were able to do this, and all you did was change a file, you have not stopped them from doing this again, and again, and again...you get the point (I hope).
Please review all these:
https://codex.wordpress.org/FAQ_My_site_was_hacked
https://wordpress.org/support/topic/268083#post-1065779
http://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
http://ottopress.com/2009/hacked-wordpress-backdoors/
Additional Resources:
http://sitecheck.sucuri.net/scanner/
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html