“How do we respond to a relative who has decided on cremation?”
Two arguments have been used against cremation.
First, we are told that it is pagan. While it is true that pagans have cremated their dead for pagan reasons, that in itself does not make the practice pagan. Pagans such as the Greeks cremated their dead because they denied the resurrection of the body, confessing only the immortality of the soul. The Greeks believed that the body is a contemptible prison for a noble soul. Therefore, death was release from that prison, and the Greeks saw no need to honor the body by burying it. We must maintain the belief of the resurrection the body. How we apply that belief in funeral practices is a different matter. Incidentally, it is also pagan to attempt to contact the dead, or to pray to them or for them. The Bible strictly forbids necromancy (Lev. 19:31, Deut. 18:11; Isaiah 8:19–20), and departed saints do not have contact with the living. They are not “looking down on us,” or involved in our lives on earth at all (Eccl. 9:5–6). They are in heaven, enjoying the glory of that wondrous place. Our hope is that they sleep in Jesus and will be resurrected on the last day. More than that, our hope is that we will be with Jesus too. Being with our loved ones again is strictly secondary to being with Jesus.
Second, it is argued that it is a sign of God’s judgment. While you could argue that in the case of Achan (Joshua 7:25) and Saul (I Sam. 31:12), that is not true in every case. Certainly, Saul was an ungodly man, but his godly son Jonathan was also cremated.
Generally speaking, Christians have preferred burial to cremation. Certainly, that is the practice of the godly in Scripture (Abraham, Isaac, Jacob, Joseph, etc.). However, the Bible never prohibits cremation. The issue, of course, is the proper disposal of and honor for the body after death. Since Jews in the Old Testament and Christians in the New Testament believe in the resurrection of the body, they dispose of the bodies of their dead honorably and in accordance with that hope. In that connection, we do not believe that, if a Christian is cremated, he will not be resurrected on the last day. Countless saints were burned alive, torn to pieces by wild beasts, destroyed in explosions, and lost at sea. God will resurrect all of them (Rev. 20:13).
Whether our bodies are incinerated in a crematorium or decompose in the earth, what God said is true, “Dust thou art, and unto dust shalt thou return” (Gen. 3:19).
So, what should you say to your relative? (I assume that we are speaking of a Christian relative). You explain to him the biblical principle of the resurrection of the body, you encourage him to organize a funeral in accordance with that principle, and you leave the details to Christian liberty.