This is a gesture known as the Christogram, and is considered the original “sign of the cross.” The fingers are positioned to form the Greek letters ICXC, an abbreviation of the Greek name of Christ: IHCOYC XRICTOC. This gesture is ubiquitous in Renaissance images of Christ and the apostles, as well as in portraits of Saints and clergy.
The Christogram is used today as a traditional gesture of blessing by priests in the Eastern Orthodox Church.
Curiously, the same gesture is known in Hindu and Buddhist traditions as the prana mudra, a symbol of healing.
Hans Memling, Christ blessing