Paul did not speak of having faith until he had lost all confidence in his flesh. He took all his education, self-confidence, abilities, doctrines and zeal and cast them aside. He spoke of faith only after he had said, "I cannot trust my flesh." The same is true for us.
Before anyone is capable of true faith, he must come to a sense of how lost, helpless and utterly hopeless he is. We do not have saving faith until we come to the end of believing that someone or something other than Jesus can save us.
Saving, justifying faith involves submitting your life to Christ with all your heart. It includes a repentance that says, "Jesus, I have got nothing to offer You. I come to submit to Your lordship!"
In Romans 10:9, Paul characterizes saving faith as believing with your heart and confessing with your mouth. He is saying that faith is more than merely giving mental assent. Rather, it is submitting your whole life to Him — with all your heart.
In Acts 8:37, Philip said to the eunuch, "If thou believest with all thine heart . . ." and the eunuch replied, "I believe. . . ." This was not simply a mental "yes" to Jesus — he really believed with all his heart and he was saved.
In contrast, Simon Magus believed Paul's preaching. Yet he had only a temporary faith because his heart was not in it. Indeed, multitudes of people in Jesus' day believed temporarily in the name of Christ, but Jesus would not commit Himself to them because He knew their hearts were not fully committed (see John 2:23-24).
So, you ask, who is truly justified by faith? It is the one who knows he is lost and helpless and he has tried everything and failed. Now he commits his whole life into the Lord's hands — with all his heart, mind, soul and strength. He cries out, "Lord, I am Yours! You are my only hope." And he is saved!