John Chrysostom

He well says, "reckon," because there is no setting that, which he is speaking of, before the eyes as yet. And what are we to reckon? one may ask. That we "are dead unto sin, but alive unto God. In Jesus Christ our Lord." For he that so liveth will lay hold of every virtue, as having Jesus Himself for his ally. For that is what, "in Christ," means, for if He raised them when dead, much more when alive will He be able to keep them so.

Homily on Romans 11 ↗

Theophylact of Ohrid

Since Christ does not die a second time, neither do we die a second time through a second baptism (that is, for us there is no second baptism). Therefore, let us remain in the former, that is, in the death of sin, but in the resurrection of life according to God. And this we received in Christ Jesus, that is, with His help: for He Who raised us when we were still dead will all the more preserve us in life now that we have become alive.

Commentary on Romans ↗

Thomas Aquinas

Then when he says, so do you also, he reaches the intended conclusion that we are conformed to the life of the risen Christ both with respect to its death to sin, i.e., to the mortal life which has the likeness of sin, never to return to it, and with respect to living conformed to God. In regard to the first he says: so do you also reckon that you are dead to sin, i.e., as never to return to sin: let not the dead live (Isa 26:14). …

Commentary on Romans ↗