The question "Was Jesus thought to be Jewish before the Renaissance?" and its attached question as to which sources might prove this might be timeless and not depending on scientific sources, but the Bible itself. The Bible text has not changed and was known, be it in Latin or not.
Where might this topic be embedded? Taken from this discussion "Jesus for Jews".
Acts of the Apostles 1:8 NLT
But you will receive power when the Holy Spirit comes upon you. And
you will be my witnesses, telling people about me everywhere—in
Jerusalem, throughout Judea, in Samaria, and to the ends of the
Acts of the Apostles 2:14 NLT
Then Peter stepped forward with the eleven other apostles and shouted
to the crowd, "Listen carefully, all of you, fellow Jews and residents
of Jerusalem! Make no mistake about this."
Acts of the Apostles 3:26 NLT
When God raised up his servant, Jesus, he sent him first to you people
of Israel, to bless you by turning each of you back from your sinful
Acts of the Apostles 4:1-4 NLT
While Peter and John were speaking to the people, they were confronted
by the priests, the captain of the Temple guard, and some of the
Sadducees. These leaders were very disturbed that Peter and John were
teaching the people that through Jesus there is a resurrection of the
dead. They arrested them and, since it was already evening, put them
in jail until morning. But many of the people who heard their message
believed it, so the number of men who believed now totaled about
If Jewish priests have arrested the first Christians, it indirectly shows why the later Christians did not identify Jesus as a Jew, but as "Christ". Perhaps, the stressing of the words "Jesus Christ" hints at Jesus to be something out of category, not INRI as the Romans put to ridicule him. Jesus has tried to reform the Jewish religion, and then it is a question on the border to formalism whether a reformer is part of the thing to be reformed or not.
There is also a story in the bible where Jesus changes his mind from being there just for the Jews to being there for anyone. I could not find this quickly, though. It is just clear that Jesus has evolved over time, coming from a Jewish background, discussing in temples, knowing the Jewish religion very well, and by this, he was able to reform it later in a historical way.
Logically speaking, one could compare it to Luther who paved the way to Protestantism, still being Christian, still not saying that Catholics are not Christian, but with an important reform. That is why there might not have been a contradiction in Jesus being a Jew, but just being "Christ" for the Christians for most of the time, or at any point in time.
There is a different reasoning in direct opposition to the first paragraph (which takes the Bible as the source), and that is not to need any specific source at all to support the statement that Jesus was not thought to be Jewish before Renaissance.
The reason why Jesus was not regarded as Jewish before Renaissance could have been a reaction to the antisemitism in Europe from 1100 in the Middle Ages on. Most notably, there was the Edict of Expulsion which forced all of the Jews on the British islands to leave for the continent. The link also offers an overview of the years 1100-1600:
As the Renaissance was from 15th to 16th century, and as antisemitism decreased after this time so that Jews reportedly almost forgot that they were Jewish until new antisemitism arose, the answer to this question might have its foundation in the historical waves of antisemism, which were higher before Renaissance.