Ask Jesus.
You indicated there was no Scripture and asked how any could use Scripture before 400 AD. Well, here’s none other than JESUS referencing Scripture and using it. TWENTY-TWO times. Guess He doesn’t agree with you that Scripture didn’t exist before 400 AD and that none could use it before then.
The Old Testament existed in Jesus’ time. That’s fact. The inspired books of the NT existed as late as 90AD. So did many, many other writings. Did all of the early Christians know precisely which ones were and were not inspired? No. Here are some of the books/letters/writings that were floating around at the time that did not make it into the Bible because the Catholic Church authoritatively decided they were not inspired.
1.1 The Epistles of Jesus to Abgarus
2. Pseudo-apostolic (general) apocrypha
2.1 Teachings of the Twelve Apostles (Didache)
2.2 Epistle of the Apostles
- Pseudo-apostolic (specific - by Apostle) apocrypha
3.1 - Andrew -
3.1.1 Acts of Andrew
3.1.2 Acts of Andrew and Matthias*
3.2 - Barnabas -
3.2.1 Acts of Barnabas*
3.2.2 Epistle of Barnabas
3.2.3 Gospel of Barnabas
3.3 - Bartholomew -
3.3.1 Gospel of Bartholomew
3.3.2 Martyrdom of Bartholomew*
3.4 - James -
3.4.1 Apocryphon of James
3.4.2 Book of James (protevangelium)
3.4.3 First Apocalypse of James
3.4.4 Second Apocalypse of James
3.5 - John -
3.5.1 Acts of John
3.5.2 Acts of John the Theologian*
3.5.3 Apocryphon of John (long version)
3.5.4 Book of John the Evangelist
3.5.5 Revelation of John the Theologian*
3.6 - Mark -
3.6.1 Secret Gospel of Mark
3.7 - Matthew -
3.7.1 Acts and Martyrdom of St. Matthew the Apostle*
3.7.2 The Martyrdom of Matthew
3.8 - Nicodemus -
3.8.1 Gospel (Acts) of Nicodemus (aka The Acts of Pontius Pilate)
3.9 - Peter -
3.9.1 Acts of Peter
3.9.2 Acts of Peter and Andrew
3.9.3 Apocalypse of Peter - version 1
3.9.4 Apocalypse of Peter - version 2
3.9.5 Gospel of Peter
3.9.6 Letter of Peter to Philip
3.10 - Philip -
3.10.1 Acts of Philip
3.10.2 Gospel of Philip
3.11 - Thaddeus -
3.11.1 Acts of Thaddeus (Epistles of Pontius Pilate)*
3.11.2 Teaching of Thaddeus
3.12 - Thomas -
3.12.1 Acts of Thomas
3.12.2 Apocalypse of Thomas
3.12.3 Book of Thomas the Contender
3.12.4 Consumation of Thomas
3.12.5 Gospel of Thomas
- Pseudo-Pauline apocrypha
4.1 3 Corinthians
4.2 Acts 29
4.3 Acts of Paul
4.4 Acts of Paul and Thecla
4.5 Acts of Peter and Paul*
4.6 Acts of Xanthippe and Polyxena
4.7 Apocalypse of Paul
4.8 Apocalypse of Paul - other version
4.9 Epistle to the Laodiceans
4.10 Revelation of Paul*
4.11 Paul and Seneca
- Infancy Gospels apocrypha
5.1 Arabic Infancy Gospel
5.2 First Infancy Gospel of Jesus Christ
5.3 Infancy Gospel of Pseudo-Matthew
5.4 Infancy Gospel of Thomas - Greek A
5.5 Infancy Gospel of Thomas - Greek B
5.6 Infancy Gospel of Thomas - Latin
- Relatives of Jesus apocrypha
6.1 Gospel of Mary
6.2 Gospel of the Nativity of Mary
6.3 Book of John concerning the dormition of Mary (transitus mariæ)*
6.4 History of Joseph the Carpenter*
6.5 Narrative of Joseph of Arimathaea
- Sub-canonical (disputed canon) apocrypha
7.1 Shepherd of Hermas
7.2 II Clement
7.3 Diatession
7.4 Gospel of the Lord (Marcion)
- Other significant Epistles and pseudomynous writings and apocrypha
8.1 I Clement
8.2 Avenging of the Saviour
8,3 Epistles of Pontius Pilate
8.4 Letter of Aristeas
8.5 Sentences of the Sextus
8.6 Alexandrians
8.7 Revelations of Stephen
8.8 Muratonian Canon (fragment)
- Fragments of lost apocryphal books
9.1 Gospel of the Ebionites
9.2 Gospel of the Egyptians
9.3 Egerton Gospel (Egerton Papyrus 2)*
9.4 Gospel of the Hebrews
9.5 Traditions of Mattias
9.6 Gospel of the Nazaraeans
9.7 Preaching of Peter