While he was still speaking, people came from the synagogue leader’s house and said, “Your daughter is dead. Why bother the teacher anymore?” When Jesus overheard what was said, He told the synagogue leader, “Don’t be afraid. Only believe.” Mark 5:35-36

“Only believe.” It is what the Greek grammarians call a present imperative. What that means for the rest of us is that Jesus is telling this man to “keep on believing.”
This man had reached out to Jesus for help, and he then had just been informed that his daughter had died. Jesus props up his fledging faith. “Don’t stop believin’.” But rather than continuing on to say, “hold on to that feelin’” as the band Journey did, Jesus moves to action. Jesus invites the man to hold on to Him.
Like a white water rafting guide telling the people in his raft to “hold on” before a particularly rough section of water, Jesus tells this father to hold on (keep on believing) through the rapids because there is smooth water on the other side. When you are tempted to despair, keep on believing. When you’re all alone, keep on believing. When you’re in full scale battle against sin, keep on believing.