However, I need to be clear up front that, from a constitutional stand point I don't believe having it on our money in any way violates the First Amendment (if history means anything). If we were to take into consideration the context under which the First Amendment was written, it was not at all separating church from state in the sense of separating religion and state. It was simply declaring that congress (at the federal-level) did not have the right to establish a national Christian denomination. Even after the ratification of the Bill of Rights, many states had their own state religions (read "denominations") at the state-level of government.
To say that Congress (at the time) was saying religion had no place in government at all is really to just dismiss history and context all together. We won't get into that here, but if you're interested I'd recommend the following for starters:
The Christian Life and Character of the Civil Institutions of the United States - Benjamin F. Morris
Christianity and the Constitution: The Faith of Our Founding Fathers - John Eidsmore
Original Intent: The Courts, the Constitution, & Religion - David Barton
But to get back to our discussion, from a Biblical perspective, should "In God We Trust" be on government printed currency?
I would say "yes" if the following two conditions were true: (1) that God authorized the State to be in the business of printing money, and (2) if it were actually true that as a State (as represented by our civil government) it was God - the only God, the God of Christianity - in whom we put our trust.
But from a Biblical evaluation, are either of these the case? ... Let's look....
God's Authorization for the State to Print Money
You'd be hard-pressed to find anywhere in Scripture where God authorizes the civil government to print its own fiat currency (and harder still to find where the government is authorized to have a monopoly on the matter).
God has certainly authorized the civil government for our good - but with a very limited role of restraining civil evil, promoting civil good, and ensuring civil justice (Rom. 13:1-7; 1 Pet. 2:13-14). No where is the civil government given authorization to print its own currency (or especially have a monopoly on such currency). And since the civil magistrate is commanded to not turn to the right or to the left of God's law (Deut. 17:20), then the civil government should only do that which is prescribed in the law for it to do. Printing money is not one of those things. Therefore, "In God We Trust" should not appear on government-printed currency because the government should not be in the business of printing fiat money at all.
But even if we were to ignore the regulative principle of the State (that is, the idea that the civil government should only do those things which God has expressly given it authorization to do), there is then another qualification that needs to be met in order for us to truly think "In God We Trust" should appear on our currency.
Is it the Christian God in Whom the State Truly Trusts?
Never mind the fact that the government is completely ignoring the regulative principle of the State (Deut. 17:20) in too many areas to list (apart from printing money, a small list would be engaging itself in the realm of wealth redistribution, education, healthcare, waging a "war" on drugs and poverty, taxing on sales, estates, inheritances, etc. - none of which does God authorize the government to do) - never mind all that, let's look at what the government is actively sanctioning in realms with which it actually has to do - using the sword:
Engaging in political crusade wars that are by no means defensive (contra a Biblical view of war)
Legalizing infanticide which allows the murdering of over 3,000 defenseless children a day
Having a different system of justice for a government official than a civilian when it comes to violence
Refusing to penalize murders and rapists with capital punishment
The list goes on...
Any one of those would demonstrate it is certainly not the God of Christianity in whom our government (as our representatives) trusts. But all of them together make an even stronger indictment that this is not a nation that can say with any confidence that it is the God of Christianity in whom our government trusts. So why in the world should we have this bald-faced lie printed on our currency?
Well let's look at that question...
What's the Big Deal If It Is Printed on Our Currency?
First, let me be clear: I think our government (and indeed, all governments) are supposed to trust in the God of Christianity. All men everywhere (Acts 17:30), including civil leaders in their civil roles (Ps. 2:10-12), are to bend the knee to the Lord Jesus Christ (Phil. 2:9-11).
However, when our government is in complete rebellion to the Christian God (if you need one more example, compare the ruling on Obergefell v. Hodges with Leviticus 20:13), it seems it can only bring more condemnation on us as a people if we pay lip service to him (on our unauthorized government-printed currency) and yet disobey him in virtually every area of life.
Isaiah wrote of a people who did this very thing. Church, pay attention! They brought their sacrifices, they observed the appointed days and festivals, and God said to them: "I am weary of bearing them. When you spread out your hands, I will hide my eyes from you; even though you make many prayers, I will not listen..." (Isa. 1:14, 15). And Jesus said the same thing of his contemporary religious leaders, quoting from Isaiah: "You Hypocrites! Well did Isaiah prophesy of you, when he said: 'This people honors me with their lips, but their heart is far from me; in vain do they worship me...'" (Mt. 15:7-9, cf. Isa. 29:13).
This is the same Jesus who said it is better to be the son who said he wouldn't obey the father but then changes his mind and does, than to be the son who said he would obey the father but then refuses to actually do it (Mt. 21:28-32).
Mark these words:
There are six things that the LORD hates,
A lying tongue and a false witness are an abomination to the Lord in whom we say we trust. If we truly trust in Him, we need to be much more persistent in calling this nation - including its civil rulers - to repentance, before we should even think about claiming that our motto, as a nation, is that we trust in the God of Christianity.
This is especially pertinent in regard to having such a motto on the very currency that is not only unauthorized by God, but is inherently deceitful about its value.
With fiat currency, the paper in itself is not the thing of value but a receipt that is supposed to be backed by the actual thing of value (in most cases gold or silver). In its history, as carrying around bags of gold and silver could get quite cumbersome it seemed legitimate to carry around receipts that could at any time be exchanged for the actual gold or silver that the receipts reflected.
As of right now, in the United States the government is the only organization that is legally allowed to print money (receipts supposedly of value). When they print a dollar bill, that dollar should be worth a dollar equivalent in scarce resources (gold or silver, etc.).
But the government has currently authorized itself to print fiat money whenever it deems it necessary. What this does though is put more receipts out there for things of value that are not really there. In other words, the government itself is counterfeiting. It's saying this paper bill is worth $1, or $5, or $20, etc. when in fact, the amount of scarce resources to back it hasn't changed at all. And so the fiat money the government just printed is worth significantly less than what it's claiming to be worth.
And this is a grievous wickedness in God's sight:
"A false balance is an abomination to the Lord" (Prv. 11:1)
"Unequal weights and unequal measures are both alike an abomination to the Lord" (Prv. 20:10)
"Unequal weights are an abomination to the Lord, and false scales are not good" (Prv. 20:23).
Does this nation trust in God? Supposedly that's our motto, but in practice it is certainly not the case at all - at least not by judging from the things our government does.
So in reality, not only should "In God We Trust" not be on our currency, I would think at this point in our history - because the statement is so absolutely hypocritical and deceitful - it is the very last place we should seek to see its wording.
Let the atheists take it away; and let that be a spark in the church - not to call for an empty motto to be placed back on unauthorized currency - but to prayerfully seek repentance for our nation, to repent as individuals, to call everyone to repentance, including our civil leaders, and to live in light of the Scriptures in every area of life!
All Scripture is breathed out by God and profitable for teaching,