Judas Iscariat's replacement / Casting lots?

The reason is that casting lots was done in Old Testament times based on the Levitical code itself which instructed that the High Priest Aaron was to cast lots upon two goats on the Day of Atonement to determine which would be the "goat for the Lord" that would be sacrificed in the Temple and which would be chosen for "the scapegoat" (see Leviticus 16:8).

Also, the land was divided between the twelve tribes by the casting of lots (see Joshua 18).

There are also many other examples of the use of casting lots to determine God's will in Old Covenant times.

In later years, because of a bad history of the priests on duty in the temple resorting to bribery and other nefarious means to be selected for the various ministries in the temple, the issue was settled by casting lots upon the priests to determine who was to carry out the various Temple duties. This is why Zacharias was offering the incense the morning the angel appeared to him in the Holiest and told him the wonderful news that Elizabeth would bear a son who would be the forerunner of the Messiah (see Luke 1:8-9). The lots had been cast four times that morning to determine which of the priests would carry out the morning's rituals. The offering of the incense, which represented the prayers of the people, a sweet savor to the Lord, could only fall upon a particular priest once in his life, so it was a great honor to be chosen and many priests lived their whole lives ministering twice yearly in the Temple and died without ever having set foot in the sanctuary itself.

The method used by the Jews was to have the priests stand in a circle. One priest would be chosen to step forward and he would be the priest upon which the counting would begin. The officiating priest would then call out a number and each priest would then hold up either one, two, three or more fingers (it was unlawful in Israel to count the people). The counting would begin with the priest who had been chosen for that purpose and the officiating priest would then begin counting fingers until he reached the number chosen and that priest would be the one chosen for that particular duty. This was done four times each morning to determine the various duties, and numerous times during the feast days to determine which priests would perform which duties.

So from an Old Covenant sense, the casting of lots to choose the priest/apostle was totally in keeping with Old Covenant practice.

Under the New Covenant ordering, every believer is chosen and anointed by God and is therefore qualified to serve God and minister to the people. The casting of lots to determine who among God's people should minister is no longer necessary, we are all ministers of God's Temple. I believe it is the Baptist doctrine referred to as "the priesthood of the believer."

In Christ,
Pilgrimer