Sometimes the turn from truth to error is slow.
It is possible to stand back and survey this in the history of a local church, Christian organisation, denomination, or movement. The original confessional foundations become increasingly marginalised. What the founding generation thinks will guarantee long term fidelity to the gospel is eventually displaced by the decisions of future ones. It need not happen with the direct denial of particular doctrines (although that does occur) it can also happen by the relativising and reinterpreting of the very concept of confessionalism.
Sometimes the turn from truth to error can be very rapid.
In fact this is the speed at which error works in the New Testament. Consider the mess at Ephesus that Timothy has to sort out, the trouble on Crete that Titus is sent to deal with, the doctrinal confusion at Corinth, the secession of false prophets that John addresses in his letters, and the necessary warnings given in 2 Peter and Jude.
Why did error take hold so soon?
There are several reasons for this. One is Satanic activity, another is God's providential testing of his Church, a third is immaturity. But there is one reason given that ought to make us think very hard. False teaching of the highest level makes inroads into gospel churches because it is just so attractive.
Paul makes this point in Galatians 1:6-9. The Galatians have fallen for a better orthodoxy:
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel--not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
Why would you want to believe a lie? You wouldn't would you if you knew that it was a lie. But the lie has come to you dressed as "a different gospel." It presents itself as good news. In fact as better news than the apostolic gospel. That, at a human level, is why error is preferred to the truth.