I'm reading the Cambridge history of medieval europe and it observes quite frequently that people had literal real fears of the afterlife, and that the word-is-my-bond thing had some immediate currency. Popes were able to issue directions to kings and dukes, which required them to do things, and without Stalin's "where is the pope's army" reality, they accepted the direction. [I personally suspect that the ability of the pope to use religious fervour to call on OTHER kings and dukes for an army probably heavily influenced things, but it would be wrong to deny the religious aspect. Life was short and brutal, and there was no evident tolerance for athiesm, at least as recorded in this kind of history. It took the later rennaisance and reformation for aspects of roman/greek atheism to enter the conversation, and christians were just as likely as any other faith to deny truths in other written works and destroy them]

A belief that a kiss represented a bond, a contract, that the kiss itself was bound into the agreement, and represented a material part of the agreement does not surprise me. Nor, that it forms a bond of fealty, and to disrupt the ceremony required acceptance of a change in law.

Remember this is a time where at least for the nobility, to swear on oath you didn't do something was in some circumstances sufficient to be found not guilty. High status people did embarrasingly public penance to get out of excommunication.