Any group can be confident and secure within its own confines. Even radical and extreme groups can encourage themselves to observe and have confidence in subjective truth. Luke wants his readers to know why they can have confidence in the objective truth of the gospel. The world is in dire need of salvation and security. Luke's gospel is written to promote security in the face of the world's insecurity.
People try to find security, but they find it in the wrong things.
Luke's gospel exposes false grounds for security and he shows that security is found only in Christ. As we have certainty about Jesus, we will have certainty in life. It is a message that all nations, all tribes and all tongues still need to hear.