We must first understand that the body of flesh and bone is not what will be risen in The Resurrection of believers.

1 Cor. 15:42 So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption:
1 Cor. 15:43 It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power:
1 Cor. 15:44 It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body.

The second is that our body is but dust.

Gen 2:7 And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

Dust and ashes are really the same thing; just one is formed more rapidly by fire.

Gen 18:27 And Abraham answered and said, Behold now, I have taken upon me to speak unto the Lord, which am but dust and ashes:

So cremation is nothing more than the burial of the body after causing it to return to the dust state rapidly by use of fire rather than the slow process of normal decay.

As believers. we should see nothing wrong with this or any form of burial as long as the body is not given more importance than God has taught us in His Word.