It can help us understand the reference to measuring the temple by recalling when something similar is mentioned in the Old Testament. In Ezekiel 40–44, the temple was measured and in Zechariah 2 the city of Jerusalem was also measured and the purpose of those measurements was to show God’s care and protection of Israel and the future prosperity of the city of Jerusalem. So, the instruction to measure the temple here is an indication that God intends to protect his people and prosper them whatever their current circumstances.
What does John mean here by the temple of God? It is unlikely that he means the temple in Jerusalem because most commentators think this book was written twenty years or so after the city was destroyed in 70 AD. Having said that, John could have seen that temple in a vision and be saying that the temporary temple in Jerusalem was a picture in some ways of another temple, the church.
Connected to the temple that John sees are two courts. The one that he is to measure is the inner court whereas the other one, which he is not allowed to measure, is the outer. (There had been an outer court in the temple at Jerusalem where Gentiles could gather, but they were not allowed into the inner court.) Those in the inner court in John’s vision are beyond the reach of the Gentiles whereas those in the outer are under attack from the Gentiles, indeed under strong attack. It is not difficult to see here a picture of the church triumphant (inner court) and the church militant (outer court).
John draws attention to an altar in the inner court. If he is using the Jerusalem temple as a model, then the altar found in the inner court was the altar of incense (rather than the altar on which sacrifices were offered, which was in the outer court). Incense typified prayer, and it is likely that by this reference to the altar John is stressing the reality that prayer is accepted by God and is connected to his actions. This altar in God’s presence is a pointer to the reality that the prayers of the saints are made perfect in his sight by the intercession of Jesus.
John also says that the period in which this opposition will last is three and a half years (42 months), and this is the same length of time as to when the two witnesses will function (1,260 days is 42 months multiplied by 30 days). This reminds us that God is in control of time, and the period here refers to that between the two comings of Jesus. God allows the opponents to trample his people, which is often hard to understand, but it is also good to know that God remains in charge.