The answer is that they had Christ in person! (With the exception of Paul, who met Christ directly after His resurrection). Nobody needs any written evidence of s/he has the actual person in front of them. John wrote clearly about this experience.They were Christians before the Gospels and Epistles were written. If they can be Christians without those things, why do you propose the Bible as the ultimate source if they hadn't access to it as the NT hadn't been written?
It's not a trick question, I would like to know.
1 John 1:1-3, "We declare to you what was from the beginning, what we have heard, what we have seen with our eyes, what we have looked at and touched with our hands, concerning the word of life—this life was revealed, and we have seen it and testify to it and declare to you the eternal life that was with the Father and was revealed to us—what we have seen and heard we also declare to you so that you also may have fellowship with us, and truly our fellowship is with the Father and with his Son Jesus Christ."
John 20:26-29, "A week later his disciples were in the house again, and Thomas was with them. Though the doors were locked, Jesus came and stood among them and said, “Peace be with you!” Then he said to Thomas, “Put your finger here; see my hands. Reach out your hand and put it into my side. Stop doubting and believe.”
Thomas said to him, “My Lord and my God!”
Then Jesus told him, “Because you have seen me, you have believed; blessed are those who have not seen and yet have believed.”