Albert Barnes Commentary Romans 16:4

"who for my life laid down their own necks; unto whom not only I give thanks, but also all the churches of the Gentiles:" — Romans 16:4 (ASV)

Who have for my life. In order to save my life.

Laid down their own necks. To lay down the neck is to lay the head on a block to be cut off with the axe, or to bow down the head as when the neck was exposed to be cut off by the sword of the executioner. The meaning is that they had hazarded their lives, had exposed themselves to imminent danger, to save the life of Paul. On what occasion this was done is not known, as it is not elsewhere referred to in the New Testament. However, since Paul lived with them (Acts 18:3) and was often persecuted by the Jews, it is probable that he refers to some such period of persecution when Aquila and Priscilla took him into their house at the imminent hazard of their lives.

All the churches of the Gentiles. All the churches that had been founded by the apostles. They felt their obligation to them for having saved the life of him who had been their founder and who was their spiritual father.