‘Finally, brethren, rejoice (‘farewell'). Be perfected; be comforted; be of the same mind; live in peace, and the God of love and peace shall be with you.'
With this thought he moves on to his farewells. He still sees them as ‘brothers and sisters' (brethren), and bids them ‘rejoice' (while literally saying ‘rejoice' some translate as farewell, seeing it as possibly being a little like our ‘cheers', i.e. ‘be of good cheer'). His main thought is that they might be joyfully responsive. He then exhorts them to grow towards full maturity, towards perfection, to enjoy God's encouragement and comfort, to be like-minded and in unity, and to live at peace. Thus will they ‘do no evil' (2 Corinthians 13:7), and reverse the trends that he fears have arisen among them (2 Corinthians 12:20). if they ‘do no evil' all his disagreements with the church will cease, for it their evils that he is concerned about. The evil of rejecting his Apostleship, the evil of all the sins of which he has had to accuse them. Then the God of love and peace will be with them. For how can they know such a God if they do not live in love and peace?
Remarkably this is the only New Testament reference to ‘the God of love', while ‘the God of peace' is more common. It suggests that Paul is using the phrase here specifically in order to encourage love among them, the love that is so lacking (see 1 Corinthians 13), love that also results in peace.