Well, it gets complicated, and that’s not an easy question. It’s a mystery, in fact. Sometimes our substance gets incorporated into the bodies of others: sometimes because we get completely recycled if we’ve been dead long enough. Surely many people have died long ago and decomposed where crops are now being grown.
And what about people who were born with missing limbs or organs, or who died in childhood?
And what about the people who were immolated on Sept 11?
And there is the matter of donors: people bleed and receive donated blood (unless their religion forbids it). I have 2 dental implants and have received donor bone in my jaws. What Christian would refuse to donate organs or blood because he needs it for the next life? I ultimately believe that God knows what he is doing, and that there won’t be a bloody mess on the day of resurrection. I do believe that we will be given flesh from God’s original creation, that is transformed.
As it says in 1 Cor 15:
51 Behold! I tell you a mystery. We shall not all sleep, but we shall all be changed, 52 in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised imperishable, and we shall be changed. 53 For this perishable body must put on the imperishable, and this mortal body must put on immortality. 54 When the perishable puts on the imperishable, and the mortal puts on immortality, then shall come to pass the saying that is written:
“Death is swallowed up in victory.”
55 “O death, where is your victory?
O death, where is your sting?”
At the end of the day, it’s your own decision about the kind of burial you want. What matters is that it’s a service of Christian burial.
(btw, today is the birthday of Wolfgang Amadeus Mozart, who was buried in a mass grave. Very few people even attended his funeral.)