https://www.mljtrust.org/sermons-online/acts-2-46-47/the-divided-man/

What makes any person a Christian? Singleness of heart. Listen to the sermon “The Divided Man” as Dr. Martyn Lloyd-Jones works from Acts 2:46–47 and explains how the early church manifested unity and singleness of heart because they each had been unified individually. The main effect of the gospel, when it is believed, is to simplify and unify the person. Only the Biblical view of humanity really explains the state of the world and the condition of humanity. The world is busy with its learning and understanding but there is one question all must face: “How can I stand before God and be just with Him?” The gospel simplifies the problem and the answer. All the problems in the universe have risen because humanity has lost the face of God. They are sinners under His condemnation. How does one face death? How do they stand with God? How are sins forgiven? The Christian is someone who sees the one problem that confronts everyone because they have been given the one answer. The Christian is no longer seeking; they are found. They are no longer swayed by different views, ideas, and schools of thought. They have found the answer to the one great question in this one blessed person. The answer is Christ.