Religion never declines. Instead it goes to one from another. There are 2 main streams of it;
1) Those who believe with faith that nothing serious would happen after physical death
2) Those who believe with faith that something serious might happen after physical death
Modern education gives the delusion that we can assume 1) as the truth, as today's people are educated with the mindset that it's pretty safe to ignore all kinds of claims if they are unsupported by scientific evidence.
1) however can hardly be proven. It's just another form of faith. It is a religion as long as it is a common faith shared by a large group of humans.
It's in fact a deception to try to portray the picture that we should only believe things which can be evidenced. In reality, humans rely on faith in witnessing to reach a truth instead of relying on the so-called evidence.
Do you have the evidence that the earth is actually revolving around the sun? Do you have the evidence that black holes exist?
99% humans don't have the evidence before they treat them as the plain truths. Even in the case of science, we have to rely on putting trust in a small group of direct witnesses (those we call scientists) to get to a truth.
While truths other than scientific truths may not be evidenced at all. Say, human history as a whole can hardly be evidenced. For an example, Jewish historian Josephus wrote a serious of books commonly referred to as Josephus' works. Please show us which book/chapter/section are actually supported by evidence.
Similarly to the question that whether life will continue after death, it can hardly be evidenced. One has to either believe with faith that it exists or believe with faith that it doesn't. There's no middle ground there as a third option as it concerns one's life. Whenever one says that "I have no religion" it only means that "my religion goes to believing that nothing serious would happen after death".
A 100 years old atheist lying in bed with the doctors telling him that he's going to die soon, he can still have a peace of mind not because he has no religion. It is in his religion that nothing serious would happen which gives him the sense of a peace of mind.