"... Christianity declining in the US and other advanced / technological /educated nations (particularly among younger generations)."
Calling the U.S. a Christian nation is a bit of a stretch.
A Christian is known not confessing as such - what one's lips says one is - but where one's heart is. Jesus said so as much in
Matthew 15:8,9 NIV:
8 “ ‘These people honor me with their lips but their hearts are far from me.
9 They worship me in vain; their teachings are merely human rules.’ ”
Of course, Jesus was referring to Jews of his time, not to Christians for Christianity was in its formative period then. But Judaism was God's religion at that time (John 4:22) and, therefore, his words would apply to Christianity, the successor religion, in foresight.
Many doctrines of the so-called modern Christian church nowadays are not even taught in the original Christian Apostolic church. However, that is beyond the scope of this post.
The decline couldn't come soon enough. It will continue until counterfeit Christianity is finally eliminated, not only from statistics, but, as Biblical prophecies indicate, from the face of the earth - after which, everyone living would gather together in Christ.
"Wherever the body is, there the eagles will be gathered together." -
Luke 17:37 New KJV
In Jesus' illustration of the wheat and the weeds (Matthew 13:24-30), weed Christians were planted in amongst the wheat Christians. They were allowed to grow together - possibly because even the weed Christians may exhibit Christian-like characteristics to some extent - until the time of the harvest at the conclusion of this system of things when the weeds make themselves known beyond any doubt, are separated and then burned at the furnace.
True Christians have never been in the majority - no, not in Jesus time, no, nor ever (Matthew 7:13,14). Not by a long shot. True Christianity will never constitute a significant percentage of the U. S. population nor any country's population, for that matter - not in this system of things.
Self-confessed Christians will eventually get tired of their hypocrisy and dropped out completely - that's the statistic registered in the Pew survey - for it is not easy to be a Christian. The Christian road is the road less traveled in the two roads mentioned by Jesus at Matthew 7:13,14 mentioned above for it is not easy to do God's will for true Christians will be persecuted by and large.
True Christians are known by doing God's will - however challenging it may be to do. Jesus said:
Matthew 7:21-23 NIV:
“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven . Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’
The fact that those self-identified Christians have fallen by the wayside is added proof that they are not true Christians anyway and they should have never been counted as Christians in the first place.
When finally the separation is over, then Judgment Day will begin in earnest - or has it already began?
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Isaiah 65:22 For like the days of a tree will the days of my people be