The apostle John was dealing with incipient Gnosticism, an error that was resulting in many false claims to know God. That ancient problem, people making claims where no reality exists is still with us today. Here are some of the reasons for that.
1. Sinful pride that claims more and better than is true.
2. A watered down Christianity that is popular in many circles today.
No matter what lifestyle is being lived by the person making the claim, tolerance says that the claim has to be accepted as true. Sadly, Scripture indicates that nothing could be further from the truth.
3. A false gospel that puts a schism between justification and sanctification.
Justification and sanctification are not the same thing, and the confusion of the two results in an unsound doctrine of salvation. However, the Bible clearly teaches that once justification occurs, sanctification is its necessary result. To divide the two doctrines to the point that professions of faith can be made and assurance is given no matter if sanctification occurs is to ultimately preach a defective, less than biblical gospel.
4. False claims can, in our day, often be laid at the feet of the false teachings of the word of faith movement.
Any "gospel" that says, "Come to Jesus and get whatever you want," rather than, "Leave all and follow Christ," is a false gospel that leads to multitudes of false professions of faith.
5. The elevation of tolerance to the highest virtue of the land.
Tolerance is too often misdefined today, and this new definition means that to even question a profession of faith, no matter how evil the lifestyle of the person making the claim, it intolerant.
6. The truth is that we, as believers, want as many people to go to heaven as possible, and so we are tempted to accept any profession of faith as being real.