An excerpt from "Desiring God.org" entitled, "How Shall People Be Saved? Part 1" by John Piper, states: "[Romans10] Verse 14: "How shall they call upon him whom they have not believed?" You might answer, well a lot of people call on the Lord in emergencies who don’t believe on him. The two most common times for hearing the name of God or of Jesus Christ outside the a religious community is when a person hammers his finger or gets in a serious car accident. These "calls" are not from faith. They are from anger and emergency. There is no true love to Christ. He is just a skilled paramedic who can just as well disappear into the night after he has bandaged me up.
But Paul clears up this ambiguity for us very quickly. In fact, he has already done it. The calling he has in mind is a calling on Jesus Christ as Lord – our Lord, not the stranger who shows up to get us out of a jam and then disappears into the night. Romans 10:9 makes this clear: "If you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved." So the calling that saves is a calling on Jesus as your Lord. This is why Paul says, "How can you call on the one you haven’t believed." Until you believe in Jesus as Lord, you can’t call on the him as Lord." This article may be viewed at: How Shall People Be Saved?/desiringgod.org
Reference: Romans 10:14