“Is Christ truly mine?” This is the most important question a soul can ask. First published in 1658 by Scottish minister William Guthrie , The Christian’s Great Interest stands as one of the most enduring and beloved works on assurance of salvation . It was the only book Guthrie ever wrote , but it has rightly earned a lasting place among the classics of Puritan and Reformed literature. Guthrie tackles two of the most critical spiritual questions a person can face: How can I know whether I have a saving interest in Jesus Christ? - If I do not, how can I obtain it? With clarity, urgency, and pastoral warmth , Guthrie explains how our salvation is not hidden in mystery, but is discernible through Scripture . Drawing from verses like “Examine yourselves whether ye be in the faith” and “Give diligence to make your calling and election sure” , Guthrie lays out a biblical roadmap for self-examination , offering hope and direction for troubled souls. "It is not a vain thing, because it is your life… the one thing needful." — William Guthrie ✨ Why This Book Still Matters: Deeply relevant to both seekers and seasoned Christians - Recommended by John Owen , one of the most respected Puritan theologians - A rare combination of evangelistic appeal and doctrinal precision - Anchored in Scripture, yet accessible to the everyday reader - A spiritual diagnostic tool for anyone struggling with doubt or longing for assurance Whether you are wrestling with fear about your standing before God, guiding others through questions of faith, or simply want to deepen your grasp of gospel truth , this book will serve as a steady and grace-filled companion. William Guthrie (1620-1665) was a Scottish Puritan minister and author. He was the first minister of Fenwick parish church in Ayrshire, Scotland. He is known primarily for his book on assurance, The Christian's Great Interest.