What do these two have in common? - that neither is defined. Each of the terms is, therefore, entirely and literally meaningless. Until a term is defined, nobody can know anything about it.
I refer to "income" as used in 26 USC, ie the lawbook of Federal taxes, and in the 16th Amendment which enabled its Subtitle A, those on income. And I refer to "God" as used in the Bible, ie the Christian holy book - of which the first part is that of Judaism and the kinda maybe subsidiary holy book of Islam. The pre-eminent one, the Q'ran, has a different name - Allah - but as far as I know doesn't define the term either. That covers over half the human race.
Given that virtually every American believes there is an income tax and that half the human race believes the biblical God exists, these omissions are non-trivial.
Let's take a look first at "income". Some years ago I produced this paper on the subject, after corresponding with Professor Jonathan Siegel of GWU, who had published a lively web page to prove that everything the IRS does is kosher. He explained that when the law uses a term, its "ordinary, common meaning" suffices as a definition; only when its use differs from that meaning, or of course when there is none or are several, does it need defining in the law.
My paper showed that "income" does not have an ordinary common meaning, in the contexts of everyday speech, legal speech, political speech or judicial speech; and therefore that a definition within tax law is certainly required.
Yet, there isn't one. So, for that reason as well as several others named here, there is no valid law to tax it.
Before breaking out the bubbly and brushing off the IRS, however, beware that what the written law has failed to specify, the government's court system has supplied. Even though Article 1 Section 1 reserves all law-making to Congress, the Judicial Branch has stepped in to repair the omission. My LewRockwell article 1789 reasoned that such a rescue was always intended, by the founders, and so that "America's government has... actually been an oligopoly of lawyers." In other words, the definition missing from Amendment 16 and 26 USC (A) doesn't matter in practice. The FedGov will do what it wants to do. The written law is just a pretense, to keep the peasantry quiet. The whole superstructure of elections, lawbooks and debates is a blind, an elaborate myth.
Then consider "God", and my text is taken from Genesis 1:1: "In the beginning God created the heavens and the earth." The first thing to notice is what isn't there: it has no by-line. It doesn't say "This is Moses writing, and here is what I've concluded..." -though it may have been; it just doesn't say. The author also doesn't say on what basis he makes his sweeping statement; he just hands it down, as dogma. This is repugnant to my mind.
Now take his first three words: "In the beginning" - was there a beginning? How do we know? How did this author know? The Universe may have existed for ever as in the Steady State Theory, which will come back into vogue if an alternative explanation is found for the Hubble "red shift", or perhaps it did suddenly appear from nothing; the term "beginning" relates to time, and time is a dimension that requires moving stuff; a pendulum, or a spinning electron. "Before" there was stuff, there could have been no "time", and so "before the beginning" cannot mean anything. Does that mean the "beginning", too, is meaningless?
At CERN and elsewhere, scientists well paid out of tax money are busy investigating the first few microseconds of the alleged Big Bang. Perhaps there was one; if so, it may not have been the first (the universe may have been oscillating, for ever.) But they don't know, even though they have the very latest tools of investigation. So how did Moses know?
His answer might be: I know it because God told me so. But that begs the question; he is making an introductory statement about God to help satisfy the reader He exists, so he cannot validly assume that existence as part of his reasoning. If your conclusion is built in to your premise, proof is rather easy. And wholly false.
The text continues: "God created the heavens and the earth" - ie, everything, the Universe. So where was God, before everything that exists, existed? Did He create Himself? Perhaps so, but in that case the statement of Gen. 1:1 adds zippo to human knowledge. If God can create himself, so can the universe; perhaps "God" means, in fact, the universe. Without a crisp definition of the meaning of the term, nobody can ever know.
I cannot recall anywhere that it offers a definition. There is page after page of what God allegedly said and thought and did (which suggests He is imagined as some kind of super-human) but not a single verse to say what He IS. Perhaps the closest it gets is in Exodus 3:13, where Moses allegedly asks God face to face what His "name" is. The answer: "I am that I am." A nonsense phrase, devoid of meaning, probably designed only to promote awe.
Conclusion: since the term is, like "income", undefined it's impossible intelligently to discuss the subject any further - for example, to enquire whether or not this undefined entity exists. Hence: until that definition is stated, it does not. Theistic religion is as much nonsense as the myth that US law taxes what people earn.
What of these two myths in the coming zero government society? - remember that the ZGS will not happen until everyone has learned to think rationally, and hence reject government for the destructive myth that it is. That vital habit of thought will lead most people also to reject religious myths, like the one discussed here; not as a prerequisite for the ZGS, rather as a consequence of its birth. And as for the wickedly deceptive "income" tax, they will all disappear, those properly written into law as well as this one, the biggest, snuck in through the back door by the Judicial Branch.