Why is the Millennium a time of testing?
The Millennium is a time of testing because only believers enter it—Gentiles who believed during the Tribulation under the 144,000's preaching and the saved remnant of Israel. The thousand-year reign serves as an introduction into eternity, yet it must still be another period of testing before the eternal state.
Les explains that the thousand-year reign is really just an introduction into the Eternal. Although the Old Testament speaks of the Kingdom as forever and ever, Revelation puts a thousand-year time frame on it, so there is no contradiction. The millennial reign will end with a complete change in the Creation and the Universe, yet eternity will be on pretty much the same basis as the Kingdom—there will be nations again, and Christ will be ruling and reigning from the New Jerusalem. The reason it is another time of testing is that as we come into the Kingdom, it is only believers: the Gentiles who believed during the Tribulation under the 144,000's preaching, and the remnant of Israel will be saved. Thus, the thousand-year reign is a distinct period of testing even though only believers enter it.