Kennedy's reference to our forebears serves mainly to

"Forebears" means ancestors. By invoking the memory and stature and wisdom of those who "prescribed" the "solemn oath" used to swear new presidents into office, President Kennedy was connecting his new administration with all the history and accomplishment of preceding administrations. He was hoping to emphasize a connection with values upon which the United States was founded. Hope you find this useful!

Cheers,

Azure