What do you mean when you say to save one sentient being is the same as saving all beings?
"Whoever destroys a single life is as guilty as though he had destroyed the entire world; and whoever rescues a single life earns as much merit as though he had rescued the entire world." ~ The Talmud
A single being is a microcosm of the whole universe. Or, looking at it from another perspective, the entire universe is but a macrocosm of a single being.
Going further, it is the attitude of Buddhism that emphasizes the release of all sentient beings from suffering. While this is an insurmountable task, it is something for which a bodhisattva (a mighty spiritual practitioner) continually labors, prays, and encourages. The significance and attitude of “all must be saved” is only known, appreciated, and cultivated through each individual opportunity along the way. How we go about helping one being is how we go about helping them all.
It can be as simple as helping an insect get outdoors or as ambitious as curing cancer.
Remember that Buddhism is not a tradition given to proselytizing or condemning. Saving a being means to aid in the diminishment of its suffering such that it has the opportunity to know peace and Truth.
While it’s a helpful and charitable ideal when put into practice, it is also a pacifying, love-blossoming, happiness-generating internal practice of the mind.