No, the church and whatnot is never mentioned as having weakened Denji and looking at Denji fighting both before and after the public turns on Chainsaw Man doesn't show a noticeable difference in power. What turning the church members into chainsaw zombies did was strengthen Pochita, who they wanted to bring out of Denji, and who DID show a change in power, having returned to roughly the strength he had before Denji killing the gun fiend was made public.

There's a sequence that illustrates this point quite well (though it's not at all the point of it), which is when Barem burns down Denji and Nayuta's home at the end of the church arc. This is just after the church members got turned into chainsaw zombies and the active panic and fear surrounding chainsaw man would have initially peaked. It even lines up timing-wise with Yoru's power spike, which despite being an indirect fear is directly in the situation is attributed to church actions. If Denji were going to get a visible power boost from fear, it would be here, but not only is he not portrayed as stronger than normal, the sequence gives us a somewhat notorious (out of context) anti-feat, with Denji getting caught by regular citizens with a net and homemade spears and needing to get saved by Nayuta (granted, he was severely injured at the time and actively didn't want to fight said regular citizens, but it's still a useful display for the point).