The phrase “don't be a baby” is used in everyday language, but what does that imply? It does not mean that people should put aside their childish ways, but that they are no longer a child so they should act like an adult. In the sermon on Romans 6:7 titled “Freed from the Tyranny of Sin,” Dr. Martyn Lloyd-Jones expands on his previous sermon by highlighting the same emphasis that Paul writes about in this passage. Once again, Paul is confirming that the old self has died with Christ and so the Christian has been set free from sin. They are no longer under the reign of sin and should live in accordance to this truth. Those who are in Christ are justified from the guilt of sin. Paul once again proves the assurance of salvation. While mortal bodies may still be tempted with sin, Christ’s Holy Spirit dwells within and are no longer under the rule of sin. In the final day when Christ calls the believers home to Heaven, mortal bodies will then be free of all temptation. Dr. Lloyd-Jones encourages God’s people to lift their eyes to anticipate that glorious day.

http://www.mljtrust.org/sermons/freed-from-the-tyranny-of-sin/