This is indeed from a sermon titled “Loving Your Enemies” and preached at Dexter Avenue Baptist Church on Dec 25, 1957. The source link, on the other hand, is to a sermon of the same name, preached at the same place, but a few weeks earlier, on Nov 17, 1957. Here’s a link to the former: http://bit.ly/2vu2HyC. I think there are also a few possible typos in the quote:
• “… but never forget what you have done…” should be
“… but I’ll never forget what you have done…”, I think
(also, some versions have “you’ve” for “you have”)
• “… erasing totally for his mind.” should be
“… erasing it totally from his mind.”
• “… no man can ever love his enemies.” is, in some versions,
“… no man can love his enemies.”