According to the custom of the priest's office, his lot was to burn incense when he went into the temple of the Lord.
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Cornelius a Lapide
AD 1637
According to the custom of the priesthood his lot was. That Isaiah , according to Bede, in his own course, which was the eighth in order, according to the lot which had originally fallen to the family of Abia. But mention of this course has been made in verse8; and therefore the lot spoken of in verse9 is different from the course, and more particularly limits the course. The meaning, therefore, is that when Zacharias, in the order of his course, was ministering in the temple, among the various offices of the priests the office of burning incense fell to him by lot. For because there were many priests of the course of Abia, it was appointed to them by lot what office each of them should perform in the Temple. For there were four principal offices (see Exodus xxx.)—1. To sacrifice2. To light the lamps on the seven-branched candlestick3. On the sabbath-day to place twelve new loaves on the table of shewbread4. To burn incense on the altar of incense. This fourth office, therefore, had fa...
It was his lot. The priests drew lots for the different functions to be performed in the same week; and now it fell by lot to Zacharias, to burn or offer up incense, morning and evening, in that part of the temple called the holy, where was the altar of incense: Zacharias was in this part of the tabernacle. (Witham)
See Exodus xxx. 6, 8.