If ye then be risen with Christ - The apostle in this place evidently founds the argument on what he had said in Colossians 2:12; see the notes at that passage. The argument is, that there was such an union between Christ and his people, that in virtue of his death they become dead to sin; that in virtue of his resurrection they rise to spiritual life, and that, therefore, as Christ now lives in heaven, they should live for heaven, and fix their affections there.
Seek those things which are above - That is, seek them as the objects of pursuit and affection; strive to secure them.

Colossians 3:1-4 (KJV)
3 If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God.

Where Christ sitteth on the right hand of God - Notes, Mark 16:19. The argument here is, that since Christ is there, and since he is the object of our supreme attachment, we should fix our affections on heavenly things, and seek to be prepared to dwell with him.