Romans 6:5

Paul apparently meant physical resurrection in view of what follows. He was speaking of the resurrection of the body at a future date rather than the believer’s resurrection to a new type of life with Christ (cf. Ephesians 2:6; Colossians 2:12; Colossians 3:1). This is parallel to what he said about our death in the context.

We could paraphrase "united" as "fused together." The Greek word (sumphytoi) means "grown together." Our union with Christ in His death and resurrection is the basis for our future resurrection.