The power, numbers & influence of Christianity has been declining rapidly in western 1st world nations over the past 7 decades. Why?

Education is replacing superstition everywhere.

PEOPLE HAVE BEEN TURNING AWAY FROM YHVH GOD, BECAUSE THE MORE THEY GET EDUCATED AT UNIVERSITIES THEY LEARN A FALSE PREMISE OF EVILUTION AND THEY TURN AWAY FROM FATHER GOD.
MORE WORLDY EDUCATION MEANS NO GODLY EDUCATION.

Prophecy being fulfilled.
What is happening is called the Great Apostasy, which means the Falling Away.
People are falling away from God. It is a sign that we are in the end times.
It is only going to get worse till The Lord Jesus Christ returns.

2 Timothy 3:13 “But evil men and seducers shall wax worse and worse, deceiving, and being deceived.”

2 Thessalonians 2:3 “Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;”

1 Timothy 4:1 - Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;

Power and influence: possibly.
Numbers: don't be so sure. ***Percentage of population***: yes. Total numbers: I don't think so.

Man wants to throw off God and towards the end God will allow that to prove to man what he really is.

Then comes the Judgment.

Simply because the truth has never been in the majority.

Why? Because the Bible is clear the world will get worse and worse, NOT better. Therefore, even as Jesus Christ says Himself, when He Returns will He find Faith on the Earth?

The Bible foretells a Great Falling away of the Faith will happen. This happens when the 7 year Tribulation Period Starts and Jesus does NOT come when most Christians of this generation THINKS He should have (pre-tribbers) These will fall quickly from the Faith.
And its not surprising that the whole world falls from their own Faiths all over the world, when AFTER the Tribulation Period Starts the antichrist will get on TV and say "Where is your God now?"
This generation gets a boil on their butt and they cry out to God, "Why me Lord, why me?" Just wait till the Tribulation Period Starts and all the pain and suffering that will accompany that, and see how many people from this generation stays True to their own Faith. So Few it will be pathetic.
Shortly after that speech, many will fall away from their Faith, because the world will agree to outlaw all faiths, regardless what Religion that may be.
You have heard this TRUTH from a prophet of God in reading this above. See if it does not come to pass exactly as God told me it will come to pass.

I can't speak for other countries, but for people my parents age, as soon as there wasn't the social obligation to attend church, that is, where you'd be looked down on by your community if you didn't attend, a lot of them simply stopped going and religion hasn't been any part of their lives since. this seemed to happen most in the early 1960s. Not everywhere is the same, and there's still parts of America more so than other western democracies where religion is still part of those communities. But those remaining seem to have got more extreme in their views. That gentle sort of protestantism seems to have mostly disappeared. If you look at lot of churches in places like England and Australia, there's hardly anyone under 75

Is Christianity dying? Read here: https://wol.jw.org/en/wol/d/r1/lp-e/1974685

All things must pass...