What does John 3:15 mean?

Jesus explains to Nicodemus that the role of the Messiah is symbolized by the bronze serpent Moses made in the Old Testament (Numbers 21:4–9). To save the people from death, as a result of their own sin, God had Moses construct an image of a snake and place it on a pole. When it was lifted, anyone who looked at it was cured. In the same way, Christ will be "lifted up" on a cross, and anyone who looks to Him in faith will be saved. This not only symbolizes the fact that salvation is completely outside of our own ability, but that it has to be entirely by faith. Looking at an image held high on a pole required no "work" on the part of the Israelites. All it required was faith—but those who refused to express that faith were going to die as a result of their own sins.

Jesus refers to the life being offered as "eternal." This uses the Greek word aionios, which means "everlasting," "unending," "forever," or "ceaseless." In other words, this life is absolutely, irrevocably permanent. The end result of faith in Christ is not just a rescue from death, but a rebirth (John 3:3) into never-ending life.