From “Believe That” to “Believe In”
When a suspect pulled a gun to shoot at him, Police Officer Mark Walker knew his life depended on whether the bulletproof vest he was wearing would work or not. Based on the live fire tests that he saw, Walker “believed that” his vest would work. But this time, it’s different. His very life depended on it.
While Mark knew that his vest could sustain the impact of a .45-caliber round, tonight he trusted in the vest for the very first time. In that singular moment, Mark went from ‘belief that’ to ‘belief in.’ It’s one thing to believe that the vest can save a life; it’s another thing to trust it to save your own life. 
To “believe that” is to believe the facts about someone or something. To “believe in” is to trust in or depend your life on those facts. Of course, “believe that” is important. The basis of our faith must be factual or truthful.
For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures, and that he appeared to Cephas, then to the twelve. (1 Corinthians 15:3-5, emphasis added)
These are the truths of Christianity. But, according to James 2:19, to “believe that” is not enough.
You believe that God is one; you do well. Even the demons believe—and shudder! (Emphasis added)
To “believe that” is to give assent or agree with the facts. But it’s not enough. We need to “believe in” them. We need to accept those truths. In order for us to receive the assurance of salvation, we need to move from “believe that” to “believe in.” According to Acts 16:31, “Believe in the Lord Jesus, and you will be saved, you and your household.” (Emphasis added)
L. A. County homicide detective and former atheist, J. Warner Wallace, explained the difference: “It’s one thing to ‘believe that’ Jesus rose from the dead and is who He said He was, but it’s another to ‘believe in’ Him as Savior.” 
|Me and J. Warner Wallace at the CrossExamined Instructor's Academy (2015)|
Wallace shared his own journey from being a doubter to a disciple of the Lord.
As a rebellious, self-reliant detective, I initially denied my need for a Savior, even though I accepted what the Gospels told me about that Savior. In order to take a step from “belief that” to “belief in,” I needed to move from an examination of Jesus to an examination of me. … The facts about Jesus confirmed that He was the Savior; the facts about me confirmed my need to trust in Him for forgiveness. I was now ready to move from “belief that” to “belief in.” 
What about you? Have you moved from “believe that” to “believe in”?
 J. Warner Wallace, Cold-Case Christianity: A Homicide Detective Investigates the Claims of the Gospel (Colorado Springs, CO: David C.Cook, 2013). iBooks.
 __________, Alive: A Cold-Case Approach to the Resurrection (Colorado Springs, CO: David C.Cook, 2014). iBooks.
 Ibid, italics his.