David Wilkerson emphasizes the importance of spiritual maturity in his sermon 'Growth in Grace Can Be Stunted!' He warns against being swayed by falsehoods and the cunning tactics of those who oppose us, urging believers to examine their reactions to slander and deceit. Wilkerson highlights Paul's call to grow up in Christ, encouraging the congregation to respond to negativity with love and grace rather than childishness. He reminds us that while we cannot control the actions of others, we can choose to use these challenges as opportunities for growth in our faith. Ultimately, the message is about embracing maturity in Christ and responding to adversity with a Christlike spirit.