T. Austin-Sparks emphasizes the significance of building upon the foundation laid by the Cross, highlighting the duality of hope and uncertainty in the prophetic messages of Isaiah. He explains that while God has cleared the ground for recovery and restoration, the question remains about what His people will construct upon that foundation. Sparks warns against building with worldly wisdom, carnality, and soulish appeal, which God will not allow on His foundation. Instead, he stresses that love is the essential material for building up the Church, as it aligns with God's purpose and reflects the true nature of Christ. The sermon calls for believers to evaluate their contributions to the Church and ensure they are rooted in love, which is the only thing that truly builds.