Across the world, declining or increasing?
It is almost academic since the Abrahamics have 3.5 billion followers between them - more than half the entire population of the world. This doesn't include Buddhists, Hindus and Sikhs which would bump the figure up to 4.2. So while Atheism may be making inroads in Europe and North America, it is still a massive uphill journey. There are way more believers than non-believers. I personally do not think religion will ever cease to exist - the very best we can hope for is a curtailing of it's more unethical practices such as homophobia, misogyny, apostasy, circumcision and indoctrination. I personally am not against religion per se, rather human rights abuses perpetrated in it's name. Some such as Buddhism and Jainism for example, are relatively benign.