Paul begins Chapter 3 of Colossians by referring back to what has happened to us, because of all that Jesus did for us to save us, as described in the first few chapters of Colossians. Paul uses the word “if” but clearly he means that we really have died to our old life and risen to a new life with Jesus, because of the gifts of faith and baptism He has given us.
Then, he says, we are called to keep on seeking and setting our minds on the things that are above, rather than things on earth. He is not saying that all earthly things are evil or that material things are evil and only spiritual things are good. See 1 Timothy 4:4-5. He is referring to the things our old sinful nature wants us to do, as he will describe in verses 5 ff. - things we are to battle and seek to put to death. He does not say what the false teachers at Colossae said in 2:21. Rather, we focus on the things of Christ, since Christ is our life. It is often a hidden life, as we struggle with many things and do not yet see the fullness and glory of the life to come when Christ appears again at His second coming.
And yet, the promises of God are certain, in Christ. He is the basis and power for our new life in Him. 15 times the name of Jesus is mentioned in Chapter 3 and the early part of Chapter 4 - Christ, Lord. Lord Jesus, Lord Christ. 15 times Psalm 110:1 is quoted in the New Testament, as it is here. Christ is seated at the right hand of God. That means that He has power and majesty co-equal with God the Father. He can and will keep His promises as our risen and ascended Lord. Paul knew it, because the risen Lord Jesus appeared to Him. John knew it too, as he had been with Jesus before and after His resurrection, and speaks of the glory of the Word become flesh to be our Savior, in John 1:14.
This does not mean that Jesus is stuck in heaven, though, as some churches say. He is also with us always and in a very special way in Holy Communion. It is true that the fullness of all God promises us in Christ will be seen when He returns on the last day. See, for example, 1 Thessalonians 4:17-18, 2 Thessalonians 1:10, 1 John 3:2, and Philippians 1:20-21. At the same time, the certainty of our eternal future in Christ is so sure, as we stay in Him, that Paul can say that our true citizenship is in heaven, as we wait for our Lord’s return. See Philippians 3:20-21, in contrast with those whose minds are set on earthly things (Philippians 3:18-19). See the promise of Romans 8:30, also, along with the fact that Paul pictures us as already seated in the heavenly places with Christ, beginning to enjoy that heavenly hope we have. That is our heavenly hope that helps through earthly days that are sometimes very difficult. In those dark days, we keep our focus on the things above. As Paul says, “My God will supply every need of yours according to His riches in glory in Christ Jesus” (Philippians 4:19).