Luke contrasts those who believe "salvation is only in Jesus" to some men from the Pharisees who believed in Jesus yet believe "circumcision is required for salvation."
"them" refers to non-Jewish believers who were not previously circumcised.
## to keep the law ##
"to obey the law" or "to follow the law"
## to consider this matter ##
"to discuss the difference in beliefs." That is, to debate the different salvation messages between Paul's gospel (salvation in Jesus) and the Pharisee's gospel (salvation in circumcision and law).