Jesus performed many miracles during his earthly ministry (John 20:30), but John only records seven of them. And he does so for a very specific purpose, which he shares here. The term John uses for miracles is instructive, calling them ‘signs’. A sign gives direction, pointing us toward something. And that is how John understood Jesus’ miracles. They were pointing us to the glory of Jesus (John 1:14) so that we might believe and, in believing, have life (John 20:31).
For those who have believed, remember Jesus’ words and guard against those who would seek to lead you away from simple faith in your Creator and in his son Jesus, leading you into the worship of self and denial of God. This is not an admonition against rational thought and learning, but rather a warning to not abandon faith in the pursuit of rationality.