Do you believe this to the decline of the faith in White societies is a loss for Whites and the right?

No, not the decline. It's the take over that's the issue. The hot new trend in the church is to be affirming. The church supports girls in boy scouts, transfaggotism, regular faggotism, gay child adoption, negrolatry, open borders, the list goes on.

Do you believe Leftists are correct in believing it's decline is a victory for them and a sign of them further winning

Declining faith, no, not a huge victory, but taking over the church and making it gay was a yuge victory.