Although those exact words aren't found in the Bible, the idea they express certainly is. When the Apostle Paul wrote to the Christians in the city of Corinth he told them, "I could not address you as spiritual but as worldly -- mere infants in Christ" (1 Corinthians 3:1).
A "worldly Christian" is someone who's put their trust in Christ but has not grown in their faith. They are (as Paul stated) spiritual infants instead of mature adults in their faith. Instead of seeking to follow Christ, they're still following the ways of the unbelieving world -- living by its values, preoccupied with its amusements and attitudes, and even still guided by its moral values.
But that puts them in a dangerous spiritual condition, because it means their faith will not be strong enough to withstand the temptations and struggles that come their way. Nor will they be strong witnesses for Christ, because people will label them as hypocrites and won't believe what they say about Him. Christ came to transform us from within -- but it hasn't happened in their lives.
Jesus Christ came to save us from sin's power -- including its power over our lives right now. Don't be satisfied with a shallow, immature faith. Instead, make sure of your commitment to Jesus Christ, and then make it your goal to follow Him in every area of your life -- beginning today. The Bible says, "But grow in the grace and knowledge of our Lord and Savior Jesus Christ" (2 Peter 3:18).