The Doctrine Which Caused The Most Damage

 Yes, I'm going there:)

In my opinion, it's undoubtedly OSAS as in "once saved always saved" with caveat. Obviously Reformed churches have traditionally taught "perseverance of the saints" which basically means that if you show the fruits of salvation in your life and persevere in following the Bible's teachings you'll go to Heaven, while many Baptists teach Lordship Salvation and claim that God will rather cause your death (hence "mortal" sin) than allow you to do something really bad and lose your salvation.

So it's not a new doctrine. What is new is undoubtedly the "carnal Christian" stuff or like someone described it, the idea that while Jesus Christ is your Saviour, Satan can still be your lord and master and you don't need to change your life at all (because it would be doing "works"). I even read some people claiming that you can drop your faith altogether and basically become an atheist and reject God totally or start practicing Wicca or whatever and still keep your salvation.

We can thank this teaching for so many people today claiming the name of Christ and never repenting/continuing to live sinful lifestyles. Another variant of this very pernicious doctrine which was popular in my country around mid 20th century is the belief that once your child is baptised he's automatically "elected" and will go to Heaven no matter what and that Hell basically doesn't exist.

One thing to consider is that Martin Luther had never taught such nonsense:

The Lutheran Church—Missouri Synod believes and teaches that it is possible for a true believer to fall from faith, as Scripture itself soberly and repeatedly warns us (1 Cor. 10:12; 1 Peter 5:8; 2 Peter 3:17; Heb. 2:1-3, 3:12-19, 6:4-8, etc.).

Such warnings are intended for Christians who appear to be lacking a right understanding of the seriousness of their sin and of God's judgment against sin, and who, therefore, are in danger of developing a false and proud "security" based not on God's grace, but on their own works, self-righteousness, or freedom to "do as they please."...

 A person may be restored to faith in the same way he or she came to faith in the first place — by repenting of his or her sin and unbelief and trusting completely in the life, death and resurrection of Christ alone for forgiveness and salvation.