Assuming people still give 10% of their pretax earnings as a tithe, $70k per year would equate to a total income level of $700,000 for the whole congregation per year. Further assuming, these thieves only stole a small amount each year in order to go unnoticed for 17 years, so let's say the $70k they stole is 10% of the church's total donations. That means, the church earns $700k per year, which also means total congregation annual income is $7mil.

Median income in Pittsburgh is $58k, so that would roughly equate to 120 member congregation size to make up to $7mil total congregational income.

This is totally in the realm of possibility.