In the New Testament, even if a practice is non-scriptural, it still must be limited or regulated to varying degrees by certain principles laid out in 1 Corinthians 6-11. A believer might have liberty to practice something non-scriptural, but it also might not be advantageous or the best practice. He should regulate his practice by that principle, prioritizing the best practice above a merely permitted one. Liberty is for glorifying God, not for gratifying self, even if it is lawful. Believers don't have liberty to cause a weaker brother to stumble, even in a practice where they have liberty.
Evangelicals and fundamentalists among other professing Christians today have moved the goal posts on Christian liberty. Here's how they do it. Instead of having liberty only on non-scriptural issues, they say there is liberty in at least two other ways. In saying so, they have expanded the meaning of Christian liberty in unscriptural ways. They are taking liberty with liberties in ways they don't have liberty to do so.
First, many professing Christians now say they have liberty where the Bible isn't clear. More and more belief and practice has been shifted into the unclear category. If a person doesn't want to do what the Bible says to do or wants to do what it doesn't say to do, he needs only to say that the teaching isn't clear. If someone deems some scriptural teaching to be unclear, it becomes a liberty issue. More is unclear in Christianity than ever and there is an ever growing list of liberties.
Second, many professing Christians now say they have liberty in areas that are unimportant or non-essential. They are free not to believe or practice whatever teachings are unimportant or non-essential. They can do what they want in those areas. Only essentials or important teachings are necessary or required. Everything else is a liberty.
Today, if a Christian judges another professing believer in an area that he deems unclear or unimportant, the one judging is the one in trouble. Now very often a Christian does not have liberty to judge someone else in an area or issue of liberty, which happens to be where scripture is either unclear or unimportant or not essential. That considered by many now to be unclear or not essential in the Bible is now also treated by many like it is non-scriptural. They have moved the goalposts on Christian liberty.
New Christian liberties are really faithless disobedience to scripture. These are means by which professing believer either use grace as an occasion to the flesh or they turn the grace of God into lasciviousness.