For all persons entering the teachings of the Buddha, Buddha Dharma, Refuge is a necessary sacrament, but the outward ritual —while it can be helpful to our minds— is only a confirmation of an inner disposition to learn and practice the Way.
There are many who go through the Refuge ceremony and experience no transformation, while there are others who accept the Dharma in their hearts and have no need of external rituals.
Especially for those who are practicing members of a faith tradition that may interpret Buddhist Refuge as incompatible with their dogma, I am very careful not to put them in a situation where they might feel uncomfortable. If performing a public Refuge ceremony is not prudent, it is best to accept the guidance of the Buddha, the Dharma, and the Sangha privately and inwardly.
The Mahayana sutras define Buddha Nature as true purity, true self, true bliss, and true permanence. As we remove obscurations (wrong views and afflicted emotions), we manifest enlightenment. We are all enlightened (full of light), because our true nature is perfection. My Teacher often recited this mantra: om svabhava suddha sarva dharma svabhava suddho’ham —OMAll is pure as it is. I am pure as I am.
Lord Jesus taught: “The Kingdom of God is within you.” (Luke 17:21) He did not say it might be within you if you do this or that, or it could be within you at such and such a future date. He said it is within you. It is not a mere wish, or a hope for a later time. It is an unconditioned reality, here and now. Your true nature is life, light, and love.
Lord Jesus also said: “Be therefore perfect, just as your Father who is in Heaven is perfect.” (Matthew 5:48) That is, because the Kingdom of God is within you, you can manifest perfection. In Buddhist terms, because the Buddha is your nature, and the Pure Land is already in your mind, you can manifest enlightenment.
Christians need not become Buddhists, because all beings are Buddha by nature. Buddhists need not become Christians, because all beings are Christ by nature.
There is no contradiction between the deep wisdom of all legitimate spiritual traditions, even if the external forms are different. The various expressions of truth are offered to us according to our perspectives and inclinations, that we may more easily accept them. That is all.