Here the words "they," "They," and "their" refer to Barnabas and Silas.
This word marks an event that happened because of a previous event. In this case, the previous event is Barnabas and Saul being set apart by the Holy Spirit.
The phrase "went down" is used here because Seleucia is lower in elevation than Antioch.
a city by the sea
The city of Salamis was on Cyprus Island.
"Word of God" here is a synecdoche for "message of God." AT: "proclaimed the message of God" (See: rc://en/ta/man/translate/figs-synecdoche).
Possible meanings are that 1) "there were multiple Jewish synagogues in the city of Salamis where Barnabas and Saul preached" or 2) "Barnabas and Saul started at the synagogue at Salamis and continued to preach in all the synagogues they found while they traveled around the Island of Cyprus."
"John Mark went with them and was helping them"