Now if we died with Christ, we believe that we shall also live with him; i.e. as explained with regard to the future ἐσόμεθα under Romans 6:5. The explanation there given accounts for the phrase here, πιστεύομεν ὅτι, without its being necessary to refer our living with Christ exclusively to the future resurrection.
For the continuance of God's vivifying grace during life after baptism is a subject of belief.