Immortality & eternal life might seem to mean the same thing, but they don’t – even tho’ they both describe something that won’t ever end. What matters is to look at the start of what they describe rather than the end. An immortal person – Elvis Presley, Winston Churchill, Cilla Black – didn’t always exist; they had a beginning. At the end of their life they died, but they live on – are immortal – in the hearts and minds of those who love & remember them.
But an eternal person always existed – there never was a time when God didn’t exist, there never will be a time when God won’t exist. God is the eternal I am.