These persons should try reading
2 Peter 2:20-22
20For if, after they have escaped the defilements of the world by the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and are overcome, the last state has become worse for them than the first. 21For it would be better for them not to have known the way of righteousness, than having known it, to turn away from the holy commandment handed on to them. 22It has happened to them according to the true proverb, “A DOG RETURNS TO ITS OWN VOMIT,” and, “A sow, after washing, returns to wallowing in the mire.”
GodsGrace: I believe in the scripture you posed, Peter was speaking of false teachers and unbelievers that at one time learned and knew (not or never believed) the gospel (holy commandment), and might have thought they were 'saved' and yet returned back as a lifestyle again, enticing others and in their old ways and the ways of the world. I believe this verse is not about a true believer that possesses the spirit of God within them.
APAK