who have for my life laid down - “who did for my life lay down”
their own necks - that is, risked their lives; either at Corinth (Act 18:6, Act 18:9, Act 18:10), or more probably at Ephesus (Act 19:30, Act 19:31; and compare 1Co 15:32). They must have returned from Ephesus (where we last find them in the history of the Acts) to Rome, whence the edict of Claudius had banished them (Act 18:2); and doubtless, if not the principal members of that Christian community, they were at least the most endeared to our apostle.
unto whom not only I give thanks, but also all the churches of the Gentiles - whose special apostle this dear couple had rescued from imminent danger.