A promise is a commitment to act such that something will be obtained.
A guarantee is a commitment that the thing will be obtained.
According to this (admittedly idiosyncratic) distinction, I can fail to obtain something I promised to obtain without breaking my promise. If I act in order to obtain it, but through circumstances beyond my control I don't, then I have kept my promise.
On the other hand, if I guarantee something but don't obtain it, then I have not kept my guarantee. (Though we don't really speak of "keeping" and "breaking" guarantees, do we?)
I'm looking for a distinction along these lines because we often speak of "the promises of Christ," yet we often act as though we've got a guarantee of eternal life backed by the full faith and credit of the Almighty.