Christianity and Buddhism both teaches Compassion and Wisdom and a goal...heaven/Nirvana. Both are similar. Just the path towards getting to that destination is different.
Christianity may seem like taking the longer path... like taking a bus up the hill. In Buddhism there are also many paths; Theravana, Mahayana and Vijrayana. It is said that the Theravana path is the very basis (like taking the bus) and one would not achieve full Enlightenment unless one realises the Mahayana Path. Then for the more advanced, Vajrayana (taking a jet up the hill). All seem to be heading one direction, which path suits us most is totally up to our level, karma and merits also.
If we are a real good Christian/Catholic, like mother Theresa, whom I think practices the Mahaya like path, which is compassion for all sentient beings, which puts others first, I think she is like a Boddhisattva on earth. There is no doubt she is one! But has she gained Enlightenment? Would she return to do more - definitely. Will she gain Enlightenment, I definitely she will. She can return and be reborn in a situation where she learns more, does more and base on her loving giving nature, she would easily have those same qualities and imprints to continue her journey towards Enlightenment. Who knows maybe she will be reborn in to a Buddhist path.
All is just labels what matters is what are we doing now to get to our final destination and what path we choose is a choice we make that suits our level now. And the end of the day the roots are the same = Comapssion & Wisdom!