I really do think it depends on the organization. Curtailing free speech in order to qualify for tax exempt status is just dead wrong.
But allowing the mega churches the same tax exempt status as the neighborhood church that runs a soup kitchen, food pantry has a missionary group, sponsors a Boy Scout troop and a community beautification volunteer group is equally dead wrong.
We must remember that the salaries of those getting paid to work for a church are taxed and usually any land is taxed same as any other.
I give some money to a missionary church for immigrant Vietnamese. That money I give is taxed when I earn it.
Seems to me that a tax on the Roman Catholic church just might be appropriate because of their enormous wealth. Maybe.
And we have that pesky problem of church and state. We all know that when the IRS gets it's fingers in a pie -- regulations and tit for tat is soon to follow.
I don't think it would effect attendance at all.