Christ became a man in order to help humans learn how to become sons of God, Lewis says. And in the process, he helped humanity transition from one kind of life, bios, to another, zoe. The two forms of life Lewis describes are opposites. Bios is selfish, concerned only with its own survival; zoe is selfless and effortlessly moral.
The chapter begins with a point Lewis made in Book Two: Christ came to Earth to teach mankind. Here, however, he expands on his point by reiterating his theory of the two forms of human life (a theory he’s outlined in previous chapters).
Lewis asks readers to imagine being a child and making an army of tin toy soldiers come to life. It’s possible that some of the toy soldiers wouldn’t be very happy about coming to life; they might prefer being made of tin. Indeed, if only one of the toy soldiers became real, the other soldiers might not care. The notion of one toy soldier becoming real is similar to the notion of Jesus Christ showing men the new kind of life—but with one key difference. Christ’s divinity was “contagious”—his wisdom spread across the world and inspired people to change their lives. The reason that Christ’s example was so powerful is that humans are “one,” even if they seem to be separate entities. In life, humans are connected to their friends and families through bonds of trust and love.
Christ’s teachings were like a beautiful benevolent “virus,” spreading across the world. One further implication of this point is that Christianity is more than just a personal belief in God and Christ; it’s also a connection with other people, based on mutual love and respect (as Lewis argued in Book Three). Notice that Lewis’s description of Christians’ relationships with one another parallels his analysis of the Holy Trinity in the previous chapter: Christians are separate and yet united—unique individuals drawn together by their love for Christ.
Humans must choose for themselves whether they want to experience Christ’s spiritual life, or whether they want to remain “tin soldiers.” But the hardest part of all—Christ’s coming to Earth to introduce humanity to spiritual life—is over. Humans could never have discovered spiritual life on their own.
Humans could not have discovered zoe on their own because, by default, they exist in a self-perpetuating state of bios. All it took was the “spark” of Christ’s sacrifice to teach the human race that it, too, could achieve spiritual salvation.
Lewis concludes by urging his readers not to quarrel over the details of why, precisely, humans are “saved”—whether Christ died for our sins, we are washed in the blood of the lamb, etc. Good Christians will choose a “formula” that works for them, and respect other people who choose a different formula, so long as they believe in Christ’s divinity.
Lewis reiterates his point from the beginning of Book Two: the specificities of how Christ sacrificed himself for mankind are far less important than the fact that mankind has gotten a second chance at redemption, and can achieve salvation in Heaven.