Here, "altar" is a metaphor. The Christians did not literally have an altar, nor did they offer animal sacrifices. The author means that Christians have a relationship with God that the Jews who work in the tabernacle do not know about. Alternate translation: "We have an altar, so to speak, from which" or "What we have is like an altar from which" (See: [[rc://en/ta/man/translate/figs-metaphor]])