What does Brutus believe is even stronger than any oath the men could take together ? Do you agree?

He thinks a promise among honest men is stronger than any oath.

And what other oath
Than honesty to honesty engaged,
That this shall be, or we will fall for it?
Swear priests and cowards and men cautelous,
Old feeble carrions and such suffering souls
That welcome wrongs. Unto bad causes swear
Such creatures as men doubt.

I think he has a point. An oath can be seen as compensating for a lack of resolve.