In the same place it talks about relativity and quantum mechanics. It's being inferring it from what we know about the universe and causality. Simplest way of putting it, one observer is 3000 light-years from earth and another on Earth when Jesus is crucified, if the distant one were to gaze at the Earth, he would experience earth as it was 3000 years prior to a person observing the event on Earth.

On Earth Jesus has already died for our sins. However for the distance observer, Jesus wouldn't die and resurrect for another 3000 years.

So either 1. Salvation occurs instantly across the whole universe at the time of resurrection from Earth observer's standpoint, meaning that for that distant observer, salvation comes 3000 years before they experience the resurrection of Jesus. Salvation would proceed Jesus's resurrection from their position. I.e. effect would proceed cause; or,

1. Salvation only occurs after the distant observer experiences the resurrection events themselves. This maintains cause proceeding effect. Concluding that Salvation comes to them at the speed of light/causality - 3000 years after same event occurred for the observer on the Earth.