2017: If you itemize your deductions because your situation makes that better than the standard deduction (~12k), then donating next year's tithing this year gives you even more deductions to reduce your taxable income. Like years in the past, you get to claim all charitable contributions, state tax, property tax (etc.) with no caps or limits. You were going to itemize anyway, and now you are deducting even more.
Doing it this year helps out a lot because the tax brackets for this year's taxes are higher across the board than they will be next year.

2018: Imagine you didnt pre-donate in 2017 and get that tax benefit. Now you donate in 2018 and say you decide to itemize. You can deduction all your charitabe donations, but only up to 10k of state and property tax (which is now capped). At the end of the day, you might be at or below the standard deduction which just got bumped up to 24k. You think to yourself - shoot, I could have donated nothing this year like most Joe's and still claimed either the standard deduction or about the same as the standard deducion ... why did't I donate that all last year and get a better return since this year it didn't help me at all?

And then you realize: with this new higher standard deduction a new approach to donations and taxes makes sense ... alternate this pattern every year! If you are going to itemize for one year, then itemize much more than standard deduction by double donating and contributing as much as possible on the years you itemize. The next year you take the standard deduction since donations will likely be lower (unless you want to pay tithing again) and they won make a dent in your taxes especially if other deductions have caps - so just enjoy that new juicy standard deduction and hopefully the new lower tax brackeys help out too.

I may have not explained it well, but hope that helps you see what they're saying.