The book and sword, symbolising the word of God and the saint's martyrdom respectively, identify the man as Saint Paul. The roundel in the top-left corner shows the Sacrifice of Isaac. In Paul's Epistle to the Hebrews (11: 17) Abraham's sacrifice of his son is cited as an example of faith. Rembrandt seems to have been particularly interested in Saint Paul, whom he represented many times.
The last digit of the date is damaged, but is probably a '9' and the painting belongs to a group of half-length apostles and saints from the years around 1659-60.