Jesus existed before Abraham because he was the first created being.
The hostile witnesses (the Jews who tried to kill him) understood Jesus was claiming the divine name. Jesus never corrected their understanding.
The Jews of Jesus' day understood his claim better than modern JWs do. The stone-picking response proves they recognised the divine name claim.
Jesus was with God before Abraham — that's all he's saying.
If that's all he meant, why did they try to stone him for blasphemy? Pre-existence alone isn't blasphemous. Also: the phrase is 'I AM' (present tense), not 'I was.'
The Jews misunderstood him.
In , they say they're stoning him 'because you, being a man, make yourself God.' Jesus does not correct them.
'I and the Father are one' means unity of purpose, not identity.
The Greek word is hen (neuter 'one thing'), not heis (masculine 'one person'). That's actually the Trinitarian point — Jesus isn't saying he IS the Father (that would be modalism), but that he shares one essence with the Father. And the Jews' reaction proves they understood a deity claim: they immediately picked up stones to kill him for blasphemy, saying 'you, being a man, make yourself God' (). If Jesus only meant 'we agree on things,' why would that be blasphemous? Many prophets were unified with God in purpose — none of them were nearly stoned for saying so.
Don't get into extended Greek grammar discussions. The hostile witness argument is stronger and simpler.
Read in its setting, from verse 48 to the stones of verse 59, until you can retell the exchange from memory, then read . Look up how the NWT renders and compare it with the wording in the doorstep question, so you know exactly what they will see on the page. Practise asking aloud why a claim of mere pre-existence would provoke a stoning, and leave the Greek grammar aside; the hostile witnesses make the point more simply. Expect 'the Jews misunderstood him'; rehearse turning to , where they state the charge, 'you, being a man, make yourself God', and Jesus does not correct them.