It occurred to me that the condition in 9:29 is a conjunction of negations (neither..nor..nor). Taken literally, if someone believes in God but not the day of judgment, or believes in the day of judgment but not in God, the verse does not apply to them. Some may argue that these two statements apply to modern-day jews and christians.
I had the same thought a while ago, and would add that it applies to Jews and Christians of its time too. They always professed to believe in God.
Which brings in the interpretation that I've since found more reasonable. The key clause in this verse, IMHO, is its epilogue, "until they give the Jizya, by hand, in humility." That means that the verse is talking about People of the Book under Muslim rule and protection who refuse to pay the Jizya. It is like in modern times when Federal Marshals are empowered by law to collect taxes from people who refuse to pay them, by force of guns if necessary.
It has nothing to do with their being non-Muslim. Jizya is the counterpart of the Zakah (mandatory alms) which Muslims have to pay but non-Muslims don't. Zakah and Jizya are the main revenue for Islamic treasury. 9:29 does not refer to law-abiding non-Muslims. It does not give a Carte Blanche to fight non-Muslims, as many scholars have opined who referred to 9:29 as the sword verse.