Based upon a Calvinist understanding, God has eternally and unconditionally elected some to be the recipients of His loving salvation and has equally determined (one’s perspective regarding the order of decrees is impertinent to this reality) those for whom there is no hope, even if they heard the gospel from God Himself and also could recite the gospel in every language in the universe.
Calvinists are notorious for asking the unsuspecting believer, “Why did you believe in Christ and someone else does not; are you smarter, or more praiseworthy in some way?” I asked this question more times than I can remember as a young Calvinist.
The Gospel must begin with the truth that each person must be born again and spiritually transformed by accepting Jesus and His sacrifice on the cross as his Savior. But it doesn’t end there. As a consequence or fruit of being born again, we are to go into the world as salt and light (Matt. 5:13-16), seeking to preserve against decay, disinfect against the infection of sin, and dispel gloom and depression with the light of the Gospel.
You may think nothing you do will ever amount to anything for the Lord. You have no idea what God can do through you if you will commit your entire life to Him. Take the limitations off and the natural expectations down. Let God work and see what He is able to do. Lottie Moon is one of the greatest examples of what God can do as we surrender our lives to Him.
Even the idea of believing refers to individuals since only individuals can exercise faith; I do not see anything in the passage that would indicate that this “all” is restricted to something like all people groups, including some who can be saved. I do not think the passage even hints that this “all” is different from “all” including each person previously spoken of as under sin; of which “there is no distinction.” This understanding corresponds precisely with its usage before and in the following verses.