While the Bible does not explicit say the age of the earth is 6000 years, but many notable Christians and even Christian scholars have made that calculation and came to the figure of about 6000 years. Which I agree is being used by atheists to attack Christianity. So the fire did not start with atheist ignorance but it is an often argued position for many Christians who insisted on the biblical age.

https://answersingenesis.org/age-of-the-ear...d-is-the-earth/
Quote,
At this point, the first five days are negligible. Quite a few people have done this calculation using the Masoretic text (which is what most English translations are based on) and with careful attention to the biblical details, they have arrived at the same time frame of about 6,000 years, or about 4000 B.C. Two of the most popular, and perhaps best, are a recent work by Dr. Floyd Jones4 and a much earlier book by Archbishop James Ussher5 (1581–1656). See table 1.

Table 1. Jones and Ussher

Name Age Calculated Reference and Date
Archbishop James Ussher 4004 B.C. The Annals of the World, A.D. 1658
Dr. Floyd Nolan Jones 4004 B.C. The Chronology of the Old Testament, A.D. 1993

The misconception exists that Ussher and Jones were the only ones to arrive at a date of 4000 B.C.; however, this is not the case at all. Jones6 lists several chronologists who have undertaken the task of calculating the age of the earth based on the Bible, and their calculations range from 5501 to 3836 B.C. A few are listed in table 2.

We can start a thread to see how sure we are if your position of a 6000 years old earth is correct or science is right, if you want. Here as I understand is not appropriate.