The reason why Paul often preached on the Sabbath was because that
was when Jews had gathered together to hear the word
-no this is incorrect statement, gentiles wanted and where preached to on the sabbath days.
Notice which day Paul and Barnabas used for preaching to Gentiles:
(1) Acts 13:14-15, 42-44: “But when they departed from Perga, they came to Antioch in Pisidia, and went into the synagogue on the sabbath day, and sat down. And after the reading of the law and the prophets the rulers of the synagogue sent unto them, saying, Ye men and brethren, if ye have any word of exhortation for the people, say on.”
Then Paul stood up, and spoke, preaching Christ to them.
“And when the Jews were gone out of the synagogue,
[the Gentiles] besought that these words might be preached to them [the next sabbath].”
Now since Paul was preaching “the grace of God” (verse 43), here was his opportunity
to straighten out these Gentiles, and explain that the Sabbath was done away.
Why should he wait a whole week, in order to preach to the Gentiles on the next Sabbath?
If the day had now been changed to Sunday, why did not Paul tell them they would not have
to wait a week, but the very next day, Sunday, was the proper day for this service?
But notice what Paul did.
“And the next sabbath day came almost the whole city together to hear the word of God.”
Here Paul waited a whole week, passing up a Sunday, in order
to preach to the Gentiles on the Sabbath day.
Gentiles Met on Sabbath
2) Acts 15:1-2, 14-21: Study this whole passage carefully.
Certain men had come down from Judaea to Antioch, teaching that the Gentile converts
there must be circumcised and keep the law of Moses to be saved.
Quite a dissension arose between them and Paul and Barnabas.
So it was decided Paul and Barnabas should go to Jerusalem to
the apostles and elders about the question.
At the conference at Jerusalem, James gave the decision.
“Wherefore my sentence is,” he pronounced (verses 19-20), “
… that we write unto them, that they abstain from pollutions of idols,
and from fornication, and from things strangled, and from blood.”
He did not say they should not keep the Ten Commandments. The Ten
Commandments were not in question—but only the ritual law of circumcision,
which was an altogether different law. He merely mentioned four prohibitions,
and otherwise they did not need to observe the law of Moses.
But why WRITE this sentence to them? Note it!
“For Moses of old time hath in every city them that preach him,
being read in the synagogues every sabbath day” (verse 21).
The law of Moses—the first five books of the Bible—was being taught in
the synagogues every Sabbath day. The apostles were writing only these decrees,
because Gentile converts were going to church on the Sabbath day.
They had heard God’s law read and expounded every Sabbath in the synagogues
and did not need further instructions. It shows that the Gentile converts
had started keeping the Sabbath day, and went to church on that day!
And the apostles’ letter did not reprove them for this Sabbath-keeping.
This is very significant, since Gentiles had never kept the Sabbath.
Therefore it is something these Gentiles had started doing after they were
converted under the teaching of Paul and Barnabas!