Jamieson, Fausset and Brown on Acts 10:7-8

7-8. when the angel ... was departed, he called--immediately doing as directed, and thereby showing the simplicity of his faith.

a devout soldier of them that waited on him continually--of the "soldiers under him," such as the centurion at Capernaum had (Mt 8:9). Who this "devout soldier" was, can only be matter of conjecture. Da Costa [Four Witnesses] gives a number of ingenious reasons for thinking that, having attached himself henceforth to Peter--whose influence in the composition of the second Gospel is attested by the earliest tradition, and is stamped on that Gospel itself--he is no other than the Evangelist Mark.