Even so reckon ye also yourselves to be dead indeed unto sin, but alive unto God in Christ Jesus our Lord. In the verses which follow (12-14) the apostle exhorts his readers to do their own part in realizing this their union with the risen Christ, to give effect to the regenerating grace of God.
For their baptism had been but the beginning of their new life; it depended on themselves whether sanctification should follow on regeneration, as it needs must do in order to salvation.