So Christians having Christian beliefs - such as there is only one god, probably the main Christian belief - is a big cause of some sort of mythical (almost as mythical as those pagan gods like Thor that you worship) decline of Christianity, is it? What a strange logic you have.
Far from declining, Christianity - which is already the largest religion on Earth - is growing. Outside the West, the religion is undergoing explosive growth, despite the fact that it is the most persecuted religion on the planet, with the culprits mainly being Muslims, but also a few simple Christianophobes, too.
70% of people in the West are Christians. About three-quarters of all humans in Europe, North America and Oceania are Christians, and 90% of people in South America.
In 1900, there were approximately 10 million Christians in Africa. By 2000, there were 360 million. By 2025, conservative estimates see that number rising to 633 million. Those same estimates put the number of Christians in Latin America in 2025 at 640 million and in Asia at 460 million.
By the middle of this century, there will be three billion Christians in the world - one and a half times the number of Muslims. In fact, by 2050 there will be nearly as many Pentecostal Christians in the world as there are Muslims today.
Within Christianity itself, the number of Protestants like myself is growing, whilst Catholicism is in decline.