Sentient beings are numberless
I vow to liberate them.
Desires are inexhaustible
I vow to put an end to them.
The Dharmas are boundless
I vow to master them.
The Buddha’s way is unsurpassable
I vow to become it.
These are the four Bodhisattva vows used in Zen I don't know where they came from though they are probably of ancient origin. There are many versiions of these vows. Each of the large sangas seems to put their own twist on these vows. This is the translation that the One Drop Sanga uses. There are also other completely different Bodhisattva vows used in other Buddhist traditions.
The first vow is to liberate all sentient beings. The early translations of these vows uses the word "save" instead of liberate. This give the vows a Christian feeling and is rarely used now to avoid this confusion. To be "saved" means something very different for most Christians then "liberate" means for most Buddhists. We Buddhists have our own vocabulary for the fulfillment of Buddhist Practice; liberation, enlightenment, nirvana, etc.. And then a Buddhist can never be sure of what these terms mean unless he/she has fulfilled the Buddhist path and actually experienced what these terms refer to. Christians usually have a much clearer idea of what being saved is.
From the first vow we are thrown into confusion. The task is impossible. There are essentially an infinite number of sentient beings to liberate. And how do we liberate even a single sentient being when we ourselves are not liberated? And by the way what is a sentient being? Obviously we humans are sentient beings but do you have to be as intelligent and feeling and capable of liberation as a human to be a sentient being? Are dogs sentient beings, can they be liberated or are they already liberated?
Even without a thorough understanding of this first vow we can understand that it asks us to do the simple things to save other beings. To be kind and work towards relieving others suffering even knowing that anything we do as an individual will not be permanent. Yet every action we take has effects that go far beyond the immediate result of the action. In some sense we are building a kinder happier world one kind act at a time. We all have a tendency to think "me first, me first" but the Bodhisattva path is to say, "You first."
Sometimes a Zen student is given the Koan "How do you save all sentient beings?" Like all koans we initially want to think about the question but no that wont work. Zen is all about meditation so eventually we forget about trying to come up with some smart intellectual answer and just continue sitting and then if the sitting goes deep enough we have an experience that answers the question. The answer does not come out of our normal dualistic perspective but only a non-dual perspective in which an independent being does not exist. This reminds me of the last section of Dogen Zenji's Genjo Koan
Zen master Baoche of Mount Mayu was fanning himself. A monk approached
and said, "Master, the nature of wind is permanent and there is no place it does not
reach. Why, then do you fan yourself?" "Although you understand that the nature of
wind is permanent;" Baoche replied, "you do not understand the meaning of its
reaching everywhere." "What is the meaning of its reaching everywhere?" asked the
monk again. The master just kept fanning himself. The monk bowed deeply. The
actualization of the buddha-dharma, the vital path of its correct transmission, is like
this. If you say that you do not need to fan yourself because the nature of wind is
permanent and you can have wind without fanning, you will understand neither
permanence nor the nature of wind. The nature of wind is permanent; because of that,
the wind of the Buddha's house brings forth the gold of the earth and makes fragrant
the cream of the long river. (This translation is from the San Francisco Zen Center web site http://www.sfzc.org/sp_download/liturgy/24_genjo_koan.pdf )
Just as the wind is permanent but is not felt without a fan so the great perfection which is the Universe is not experienced without practice. In this great perfection all beings are enlightened and yet most suffer in the ignorance of their true nature which as Hakuin Zenji said is "no nature."