A.B. Simpson emphasizes the importance of maintaining joy in our lives, urging believers to rejoice always and to guard against the spirit of murmuring that can lead to spiritual loss. He warns that doubt and discouragement can break our fellowship with God and hinder our progress. Simpson encourages us to protect our hearts from negativity and to remain filled with the Holy Spirit, as joy is a powerful defense against the attacks of the enemy. He illustrates that just as a thin pane of glass can protect a bird from danger, so can the joy of the Lord safeguard us from evil.

Whatever else you lose, do not lose your joy. Keep the spirit of spring. Rejoice evermore and again I say, rejoice. The loss of Canaan began in the spirit of murmuring. When the people murmured, they displeased the Lord. The first break in their fellowship, the first falter in their advance, came when they began to doubt and grieve and fret. Keep your heart from the perforations of depression, discouragement, distrust and gloom, for Satan cannot crush a rejoicing and praising soul. Be on your guard against the beginning of sin. Don't let the first touch of evil be harbored. It is the first step that loses all. Let us keep so encased in the Holy Spirit and in the very life of Jesus that the evil cannot reach us! The little fly on the inside of the windowpane may be attacked by the little bird on the outside, and it may seem to him that be is lost. But that thin pane of glass keeps him safely from all danger as certainly as if it were a mighty wall of iron.