Romans 8:10

And if Christ is in you, the body is dead because of sin; but the spirit is life because of righteousness. "the body is dead because of sin" -'THOUGH the body is dead' (NASV); 'although your bodies must die because of sin' (Wms). 'The body, it cannot be denied, is dead because of sin: the experience we call death is inevitable for it' (Gr. Ex. N.T. p. 646) 'Has the seeds of death in it and will die' because of sin. (Robertson p. 373)

'The usual understanding of the phrase is that it is condemned to die (as. consequence of sin entering the world- Genesis 3:1). It already has the 'seeds of death' in it so Paul calls it dead.'

But Paul may have something else under consideration. The body which was used as an instrument for sin (Romans 6:16), has been put to death (put out of commission) 'because of sin' (in order to stop it), by those who belong to Christ. (Galatians 5:24)

"but the spirit is life because of righteousness"- it seems that 'spirit' here means the 'human spirit'. 'Your spirit is full of life' (Mon), because you have been made righteous.

Paul's point here could be the same as in 2 Corinthians 4:16. Even though the physical body will die (as. consequence of sin), the souls of those 'in Christ' are flourishing and enjoying spiritual life.