Lukewarn can mean many things according to someone's walk. For me, I had one foot in the world and the other in God's world. I learned that you cannot serve two masters as Jesus once put it: Matthew 6:24. You have to choose which one to completely follow. In my earlier days, I was half-hearted which is lukewarmness.
Another example (and a very good one) is from Revelation 3, letter to Laodicea which means Just People or Justified. When Jesus Christ is saying cold or hot, He means be pure (cold) or be passionate (hot) about Him. Be clean as Living Water and/or be a blazing Fire for the Lord; the Light that resides within us.
Revelation 3:17-18
Instead of relying on Christ they turned and relied on their richness of things in the flesh, vanity. If you read their history, God actually turned their cold water system into lukewarm water - physically telling them that is how He sees them because of what's in their heart. They don't realize how they appear to God: pitiable, blind, naked, and poor.
God then tells them to buy gold refined by fire which is to be tested and tried in the fire for our faith in Jesus Christ, so that we may become rich in Spiritual matters and be purified in His sight. To wear the white garment which is symbolic to wearing the pureness of Christ.
Because the Laodiceans sold eye salve, God ironically tells them to buy from Him eye salve so that they may see and understand the Spiritual things of God.
These things certainly reflect those who are in the world, believing in Christ, but not following Him. This I had to repent of some years ago, and even I had no idea until I fully surrendered to Christ and made the decision to follow Him with every fibre of my being. Everything Christians do ought to be in the faithfulness of Christ because without Him, we aren't able to do anything: John 15:5.