The Greek particle an is an untranslatable conditional particle that modifies the mood of a statement. When combined with the subjunctive, it marks indefinite conditions ("whoever," "whenever"); with the optative, it expresses possibility. One of the most grammatically significant particles in the Greek New Testament.
Though an has no direct English equivalent, its presence is theologically significant. The particle signals that God's promises are conditional on faith but unlimited in availability: "whoever" and "whenever" anyone calls on the Lord, they will be saved (Romans 10:13). The gospel is universally available yet personally applied.