אין יאוש בעולם כלל

I know Pesach was already a couple weeks ago, but anyways-- in a Mrs. Smiles' shiur for Pesach, entitled "Bnei Brak Bounty", she quoted the הגדה ממעיני הישועה which quoted ר' צדוק הכהן מלובלין זצ"ל:
The birth of יצחק אבינו when אברהם and שרה were 100 and 90 years old, respectively, was totally above טבע -- they had already given up hope of having children. Why was עם ישראל created this way? To show us that "אין יאוש בעולם כלל" -- there is no such thing as despair. A Yid does not give up hope!

(For more of the context and how this actually connects to Pesach, listen to the shiur on Naaleh or on 613.org -- this is approximately 25-30 minutes into the shiur. Also, the source sheet for the shiur is here -- it's source #20.)