The early believers did not have a "New Testament" and an "Old Testament"
Most Christians (Churchians) have little knowledge about Bible canon. They don't know that it was men, not God who put together and organized the Bible as we know it. They are not told that the "Old Testament" and "New Testament" was not divided by God.
The "New Testament" books were canonized by men
The "New Testament" books: the Good News (Gospel) according to Matthew, Mark, Luke and John, the letters of Peter, James, Jude and James were written at different times. So the early assemblies had access to only some of these at a time. As a matter of fact, they could not have and did not have a "New Testament" as such. Did you know that these early believers turned to Yahusha the Anointed King based on the so-called "Old Testament"? They didn't even have the remotest idea that what they are reading is going to be called by men the "Old Testament".
Though the Early Church used the Old Testament according to the canon of the Septuagint (LXX), perhaps as found in the Bryennios List or Melito's list, the Apostles did not otherwise leave a defined set of new scriptures; instead, the New Testament developed over time.
Martin Luther (1483–1546) made an attempt to remove the books of Hebrews, James, Jude and Revelation from the canon (partially because they were perceived to go against certain Protestant doctrines such as sola scriptura and sola fide), but this was not generally accepted among his followers.
The early assemblies didn't know anything about "Old Testament" and "New Testament", they only knew "Scriptures". The separation to "Old" and "New" was men's concept to deceive you that the Law is old and is not relevant to our lives. It's a lie. Yahuah never intended to have an "Old Testament" and a "New Testament". He meant to have Scriptures.
The first printed Bible came out at the 1450s and even that was Latin.
Written in Latin, the Catholic Gutenberg Bible is an edition of the Vulgate, printed by Johannes Gutenberg, in Mainz, in present-day Germany, in the 1450s
Only later, between about 1500 and 1800, when printing was modernized did believers started to have their own Bibles. But it wasn't always like that.
Early Modern English Bible translations are of between about 1500 and 1800, the period of Early Modern English. This, the first major period of Bible translation into the English language, began with the introduction of the Tyndale Bible. The first complete edition of his New Testament was in 1526. Tyndale used the Greek and Hebrew texts of the New Testament (NT) and Old Testament (OT) in addition to Jerome's Latin translation. He was the first translator to use the printing press – this enabled the distribution of several thousand copies of this translation throughout England.
The disciples preached using the "Old Testament"
Since Paul didn't have the so-called "New Testament", obviously, the word "scriptures" stood for the so-called "Old Testament". He used the Law and the Prophets to preach about Yahusha.
Now when they had passed through Amphipolis and Apollonia, they came to Thessalonica, where there was a Jewish synagogue. Paul, as was his custom, went in to them, and for three Sabbath days reasoned with them from the Scriptures, explaining and demonstrating that the Anointed King had to suffer and rise again from the dead, and saying, “This Yahusha, whom I proclaim to you, is the Anointed King.”
Since the people of Beroea didn't have the so-called "New Testament", obviously, the word "scriptures" stood for the so-called "Old Testament". The believers were examining the scriptures ("Old Testament") to find out if what Paul preached to them from the Scriptures ("Old Testament") about Yahusha was so.
The brothers immediately sent Paul and Silas away by night to Beroea. When they arrived, they went into the Jewish synagogue. Now these were more noble than those in Thessalonica, in that they received the word with all readiness of the mind, examining the Scriptures daily to see whether these things were so.
When Paul was in Rome he was preaching about Yahusha from where? From his own letters to the Romans? Did he preach from the Good News (Gospel) of Matthew? No. He preached about Yahusha from the Scriptures ("Old Testament"): the Law of Moses and the Prophets.
When they had appointed him a day, many people came to him at his lodging. He explained to them, testifying about God’s Kingdom, and persuading them concerning Yahusha, both from the law of Moses and from the prophets, from morning until evening.
Here we read about Philip who preached about Yahusha based on book of Isaiah "Old Testament".
He arose and went; and behold, there was a man of Ethiopia, a eunuch of great authority under Candace, queen of the Ethiopians, who was over all her treasure, who had come to Jerusalem to worship. He was returning and sitting in his chariot, and was reading the prophet Isaiah.
The Spirit said to Philip, “Go near, and join yourself to this chariot.”
Philip ran to him, and heard him reading Isaiah the prophet, and said, “Do you understand what you are reading?”
He said, “How can I, unless someone explains it to me?” He begged Philip to come up and sit with him. Now the passage of the Scripture which he was reading was this,
“He was led as a sheep to the slaughter. As a lamb before his shearer is silent, so he doesn’t open his mouth. In his humiliation, his judgment was taken away. Who will declare His generation? For his life is taken from the earth.”
The eunuch answered Philip, “Who is the prophet talking about? About himself, or about someone else?”
Philip opened his mouth, and beginning from this Scripture, preached to him Yahusha.
Wait! Do you mean that the "New Testament" believers turned to Yahusha the Anointed King based on the "Old Testament"? Yes, this is exactly what I mean. In fact, there wasn't and there still isn't anything like "New Testament" or "Old Testament" according to the Scriptures.
If the early believers were able to come to the faith through reading the "Old Testament" why is it that contemporary Christians rarely read the "Old Testament", if not at all? And why is it that the disciples used the Law and the Prophets to preach about Yahusha while contemporary Christians say that the Law was abolished?