In Revelation 3:14-16 we read

To the angel of the Ekklesia in Laodicea write: These are the words of the Amen, the faithful and true witness, the ruler of God's creation. I know your deeds, that you are neither old nor hot. I wish you were either one or the other! So because you are lukewarm - neither hot nor cold - I am about to spit you out of my mouth.

Earlier today had been challenged to think "what are some modern signs of a lukewarm church or believer?".

One definition would be a people / person without a prophetic vision.
At the minimum that would express itself as failure to prepare the next generation.

But back to the passage I quoted.
Ekklesia is not a religious word. It causes confusion / is misleading when it is translated into English as church.
An Ekklesia was a gathering of local citizens to address political issues relating to local governance.

That puts an interesting perspective on Jesus's description of Himself.

Amen - literally means, 'Yes! I agree! So be it!'. Suggests the making of a decree.

Witness - someone who speaks up about what they have seen, heard, experienced.

Ruler - someone who speaks with authority.

So a definition of a lukewarm church / believer?
... someone who fails to speakup when they have the opportunity.

We have the promise of Jesus to those who doubted in Matthew 28:17-20
Because all authority has been given to Me (Jesus) you (the doubters) can go and make disciples, baptising them ... and teaching them ...