If, however, morality can change according to time and circumstances, the Church is destined to perish in the relativism of today's fluid society.
This is crux of the issue. Truth is timeless. What is fundamentally true today will be true tomorrow and will be true ten years or ten centuries from now.
What God has said is sin is sin today, is sin tomorrow and is sin forever.
The Episcopal Church, the Anglicans, the Methodist and many of the Liberal leaning churches have fallen in to this trap and are bleeding members. The Catholic Church has also seen dwindling attendance but not to the same degree.
The decline of the churches in my opinion is due to the church’s move to a humanistic approach to worship and moving away from a God centered worship.
What Pope Francis is doing with Amoris Laetitia extending humanism in to the institution of marriage and making the sin of adultery subject to individual conscience. (If the sin of adultery doesn’t bother my conscience it must be okay with God.)
Why would anyone need to go to church if all matters of sin are simply matters of personal whim?
All true.
The Catholic Church, founded by Jesus, will survive and thrive. Maybe it won't thrive here but there are places in the world where it does.
All good people KNOW that the "absolutes" are essential BECAUSE they are timeless. Most societies have had the same kinds of morality, always.
Then there are those societies where, for example, abortion is legal and moral. THAT is sad.