But back to the o/p - is Christendom generally falling into apostasy including those characterized by "fundamentalism"?
The scripture seems to indicate it.
2 Thessalonians 2:3 Let no man deceive you by any means: for that day shall not come, except there come a falling away (ἡ ἀποστασία - the apostasy with the definite article) 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;
I think there were plenty of antichrists plus dogs & pigs in the first century. Such warnings & prophecies are of course very relevant to Christians of all generation down the ages.
Paul in 2 Tim. 3 makes the same point -
1 But mark this: there will be terrible times in the last days. 2 People will be lovers of themselves, lovers of money, boastful, proud, abusive, disobedient to their parents, ungrateful, unholy, 3 without love, unforgiving, slanderous, without self-control, brutal, not lovers of the good, 4 treacherous, rash, conceited, lovers of pleasure rather than lovers of God – 5 having a form of godliness but denying its power. Have nothing to do with such people.
That last instruction is to Timothy himself. There are many such warnings in the letters.
The obvious danger for fundamentalists is complacency - we hold the truth, so the warnings don't apply to us. But do we have a zeal for the truth, for godly living, Gospel outreach, concern for the lost, etc.
And is our fundamentalist theology truly Biblical? In particular, there are many divergent views on this forum as to Biblical theology. I reject the idea of the reestablishment of the state of Israel as a physical nation in the land promised to Abraham. Such doctrines have no basis in the teaching of Jesus & his Apostles, so indicate apostasy. [Please don't argue the case on this thread - there are plenty of others!]
Other apostate doctrines may include infant baptism, a state church - which automatically includes the unconverted, modern prophets & apostles, priesthood of leaders, pop-style worship, decisionist evangelism, etc.
Whether we should argue most or many, I think the OP is right to express serious concern.