Why is Christ called "the firstborn of the dead"?

Revelation 1:5 (ESV)

5 and from Jesus Christ the faithful witness, the firstborn of the dead, and the ruler of kings on earth.To him who loves us and has freed us from our sins by his blood

Jesus died on Good Friday and dead people commonly remain dead. But on Easter Sunday Jesus triumphantly arose from the dead as evidence that he had atoned for sin and so had broken the power of death. As the one who broke the power of death, he would guarantee that many would arise from the dead after him (see also 1 Corinthians 15:20).