Origins
The first sponsors did not write cheques. They poured wine. Latin spondēre meant 'to pledge solemnly', and its PIE ancestor *spend- meant 'to pour a libation' — the act of offering liquid to the gods to seal a sacred promise. A sponsor was one who made such a pledge on behalf of another.
In Roman law, a sponsor was a guarantor — someone who stood surety for another person's debt or obligation. In the early Church, the word was adopted for godparents: a baptismal sponsor pledged to guide a child's spiritual development. The commercial sense of funding an event arrived only in the 19th century.
Latin Roots
The family that spondēre produced is striking. A spouse is someone pledged in marriage — Latin spōnsus and spōnsa were 'the pledged man' and 'the pledged woman'. To respond is to pledge back (re-spondēre). To correspond is to pledge together. And to be despondent is to have given up your pledge entirely — dē-spondēre, to un-promise, to lose hope.
Greek σπονδή (spondḗ) meant 'a libation' and, by extension, 'a truce' — because truces were sealed by pouring offerings. A treaty and a splash of wine were the same ritual act. The corporate sponsor paying for a stadium's naming rights stands at the end of a long tradition that began with wine on stone altars.