A divine figure comes down from the sky and tells you that you need to repent and become LDS.
The desert fathers recommend not accepting angelic visitations, as the devils often disguise themselves as such to gain entry into the cell of the unaware. I would go by their advice, being chastened by the fact that no angel from God would tell me to leave Christ our God by converting to a religion that sees itself as the correction Christianity like Mormonism or Islam. It's a shame that JS or Muhammad did not heed this wisdom themselves.
They show you an extensive list of sins that they say you're guilty of and give this as the reason why.
How do you react?
With suspicion.
You'd darn sure want to make sure that this divine figure is indeed of God, right?
And that's supposed to be accomplished by a handshake test
how, exactly? This is nothing but Masonic nonsense dressed up in Christian verbiage.
So why do you begrudge us that same sentiment, that of wanting to be sure someone or something is indeed of God?
Because you don't judge someone's claimed vision based on a handshake or some other 'tokens' or phrases like you're running an underground bar during prohibition-era America. Every claimed vision or message must be evaluated according to how well it conforms to the faith that has been delivered once to the saints, and those things that do not conform to it
(like Mormonism) must be rejected. The
content, not the knowledge of some specific handshake, is what matters.
From letter 41 of St. Jerome to Marcella concerning the scourge of Montanism in Rome (385 AD):
If, then, the apostle Peter, upon whom the Lord has founded the Church, has expressly said that the prophecy and promise of the Lord were then and there fulfilled, how can we claim another fulfillment for ourselves? If the Montanists reply that Philip's four daughters prophesied at a later date, and that a prophet is mentioned named Agabus, and that in the partition of the spirit, prophets are spoken of as well as apostles, teachers and others, and that Paul himself prophesied many things concerning heresies still future, and the end of the world; we tell them that we do not so much reject prophecy— for this is attested by the passion of the Lord — as refuse to receive prophets whose utterances fail to accord with the Scriptures old and new.