Isaiah 6:1 refers to a heavenly temple. My question is: Is that in the future or during the millennium? Also, there are some scholars, such as Tim Lahaye, who teach that there will be sacrifices during the millennium. What are your thoughts on these matters?
To answer your question we must back up to an assumption that you are making which isn't found in the Bible. People who talk about the millennium are generally referring to a belief that Jesus will return to earth in the future to reign for a 1,000 years. While there is one mention of a 1,000-year reign in the Bible, that verse appears in Revelation, which is written in symbolic language. In order to have a thousand-year reign, a literal meaning must be applied. But the problem is that those promoting pre-millennial theories are arbitrary in their assignment of meaning. They will apply figurative meaning and literal meaning to the descriptions in Revelation with little reason beyond what they want the passages to say. The article "The Thousand-Year Reign" covers this passage in Revelation in detail, so I won't repeat it here. In addition, there are several articles showing how Scripture has been twisted to produce a doctrine that isn't taught by God. See "Premillennialism".
"In the year that King Uzziah died, I saw the Lord sitting on a throne, high and lifted up, and the train of His robe filled the temple" (Isaiah 6:1). There is nothing in this passage to indicate that the temple Isaiah saw was different from the Temple Isaiah knew in Jerusalem. The imagery is that is God reigning from His throne. The train of the robe God is seen wearing in this vision filled the temple. In other words, the presence of God is seen in the temple. Ezekiel described a similar scene. "The Spirit lifted me up and brought me into the inner court; and behold, the glory of the LORD filled the temple" (Ezekiel 43:5).
As far as the fulfillment of this vision, we do not have to guess because God has told us. In Isaiah's vision, he is commissioned by God to bring a message to the people. "And He said, "Go, and tell this people: 'Keep on hearing, but do not understand; Keep on seeing, but do not perceive.' Make the heart of this people dull, And their ears heavy, And shut their eyes; Lest they see with their eyes, And hear with their ears, And understand with their heart, And return and be healed."" (Isaiah 6:9-10). In one sense, this vision was fulfilled by Isaiah in his day because he delivered the message given to him. Yet, it also has a deeper meaning. "Therefore they could not believe, because Isaiah said again: "He has blinded their eyes and hardened their hearts, Lest they should see with their eyes, Lest they should understand with their hearts and turn, So that I should heal them." These things Isaiah said when he saw His glory and spoke of Him" (John 12:39-41). God, through John, is saying that for the same reason the people rejected the message in Isaiah's day is the same reason they rejected Jesus.
In regards to sacrifices, the idea that animal sacrifices would be restored shows how little these "scholars" respect the word of God.
"For the law, having a shadow of the good things to come, and not the very image of the things, can never with these same sacrifices, which they offer continually year by year, make those who approach perfect. For then would they not have ceased to be offered? For the worshipers, once purified, would have had no more consciousness of sins. But in those sacrifices there is a reminder of sins every year. For it is not possible that the blood of bulls and goats could take away sins. Therefore, when He came into the world, He said: "Sacrifice and offering You did not desire, But a body You have prepared for Me. In burnt offerings and sacrifices for sin You had no pleasure. Then I said, 'Behold, I have come--In the volume of the book it is written of Me--To do Your will, O God.'" then He said, "Behold, I have come to do Your will, O God." He takes away the first that He may establish the second.Previously saying, "Sacrifice and offering, burnt offerings, and offerings for sin You did not desire, nor had pleasure in them" (which are offered according to the law), then He said, "Behold, I have come to do Your will, O God." He takes away the first that He may establish the second. By that will we have been sanctified through the offering of the body of Jesus Christ once for all. And every priest stands ministering daily and offering repeatedly the same sacrifices, which can never take away sins. But this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God, from that time waiting till His enemies are made His footstool. For by one offering He has perfected forever those who are being sanctified. But the Holy Spirit also witnesses to us; for after He had said before, "This is the covenant that I will make with them after those days, says the LORD: I will put My laws into their hearts, and in their minds I will write them," then He adds, "Their sins and their lawless deeds I will remember no more." Now where there is remission of these, there is no longer an offering for sin" (Hebrews 10:1-18).
To claim that sacrifices are to be restored is to claim that Jesus did not offer the perfect sacrifice for sin. Those sacrifices under the Old Law existed because of sin, yet they could not in themselves do anything about sin. Jesus came and offered himself as the sacrifice that takes sin away. Now that this sacrifice has been offered, there is no longer any need for animal sacrifices.