A phrase I had not really thought about much. Like so many other phrases in English they usually revolve around something, whether historical or utilitarian in everyday life. For example, the phrase, back to square one refers to an old BBC radio convention for covering soccer games. In the paper they would print a card showing the soccer field with each part of the field sectioned in blocks with numbers.
Square one meant you were in your own half with the ball.
So with that in mind lets talk about Iota.
Iota is, according to wikipedia and other sources it was the 9th letter of the Greek alphabet. However, what is significant with this letter is how Christianity changed because of it.
The addition of Iota to the word homoousios makes homoiousios. So what does that mean, and how does that change a religious movement. In 316 AD at Nicae in Turkey the battle of the Bishops of the early church revolved around understanding the nature of Christ.
For Aryans Christ was of the same substance, homoousios, but rather of a similar substance of God, homoiousios. For the “Orthodox” bishops, such as Nicholas (modern Santa Claus), homoousios was the only way to understand the trinity. The idea of the father, and the son being anything other than of one substance was tandemount to heresy.
So the Christian world, with its creeds, spun in the next few years of the 4th century on one letter, iota. So in English this phrase points to how even one letter can cause wars, exterminations, and persecution. Thus, if you have not one iota of difference it means you have a meaningless separation or distinction, your positions are the same.