Zac Poonen emphasizes that the true measure of our service to God lies not in the actions we take but in the motives behind them. At the judgment seat of Christ, the critical question will be 'Why did you do it?' rather than 'What did you do?' He illustrates this with the parable of the prodigal son, highlighting how both the younger son's selfishness and the elder son's legalism reflect flawed motives in serving God. Poonen urges believers to examine their hearts and serve out of love for God rather than for personal gain or obligation, encouraging a joyful and genuine relationship with the Lord.