In 2000 years Christianity has grown from 12 disciples to about 2,000,000,000 followers. What is the average rate of growth? If you guessed 0.95% then maybe you can move on. If something higher like 2% go the back of the line and repeat the section. At 2% there would be around 300 quintillion Christians.
If you have questions that involve compound interest (investments, loans, tax rates) do the arithmetic, before you decide.