I mean every person that the athiest movement convinces to leave religion, makes one less person donating their time and money towards the charitable initiatives of the church, = less for charity.
Proselytizing does not help anyone. If you spent as much money on mosquito netting as christians do on missions based literature we could save millions of children from malarial disease.
Lets assume that all the people who leave the church commit the same donations to secular charities instead(unlikely, but a percentage probably would), what about the lost volunteers?
Just imagine the number of Salvation Army volunteers we would lose if people stopped believing in Santa Claus.