What the New Testament actually reports is spare and anonymous. The Gospel of John alone records that, after Jesus had died, "one of the soldiers pierced his side with a lance, and at once there came out blood and water" (19:34). The soldier is not named, not described, and not said to have converted. A separate detail in the Synoptic Gospels has a Roman centurion, watching the crucifixion, exclaim that "truly this man was the Son of God", but the Synoptics give this to a different figure and do not connect him with any piercing.
Everything beyond this belongs to later tradition. The name Longinus is not in scripture. It is generally explained as formed from the Greek lonche, meaning lance, that is, the man was named after his weapon, or after his function in the story, rather than the reverse. This is the kind of naming a devotional tradition supplies when a scriptural figure is anonymous and a cult requires a name.
The atlas therefore keeps two things distinct: the unnamed soldier who is a datum of the Fourth Gospel, and Longinus, the named saint who is a construction of the apocryphal and hagiographical tradition built upon him. The lance he is said to have carried is venerated as the relic known as the Holy Lance.