What can I use as a replacement for cumin?

The problem is that a lot of substitutions are based on what you have on hand.

For instance, I have both liquid smoke and smoked paprika, both of which would add that smoky quality that people associate with cumin.

We also need to consider what other ingredients are going into the dish. I have spice blends that contain cumin in varying degrees (garam masala, taco seasoning, a rub for ribs), and if they contain other ingredients that I might be using anyway, I'd consider replacing all of them with the spice blend.

Chili powder and things like chili or taco spice mixes will normally have cumin as an ingredient. Of course they have other ingredients as well, but they might work well in a lentil soup.

In India, when we make lentils we finish it by tempering with cumin or black mustard seeds. Infact these two are used interchangeably with many other dishes and gives the right kind of flavour required for any such dishes.

Incase you don't have even that on hand, flavour your dish with othe spices like chilli powder and garam masala and it should turn out just fine. Many spice mixes contains cumin powder so if you could get your hand on any of the mixed spice powders, it should do the trick.

Coriander might give you the right general sort of flavor, but I think it's darker and heavier than cumin (I'm struggling to describe this well, obviously). Perhaps mixed with paprika to brighten it up a little?

If you've got some, give it a sniff and see; it might work for the desperation, if nothing else.

My first instinct was curry powder and perhaps some garam masala.

Not really like cumin, but also a classic combination with lentils and onions - an indian twist, so to speak.
(Amp up the heat with chili and top with a dollop of yoghurt, if you like.)