‘But God has called us in peace.' When God called us it was essentially in the sphere of peace, peace with God and peace from God. God does not seek to bring His people into a position of antagonism and conflict, nor does He want it. It may arise because of the nature of the unbelieving, but it is never God's aim. If the result of trying to maintain the marriage is conflict on religious matters which results in the partner walking out then he/she need not feel burdened at his/her failure to maintain it because of their partner's behaviour. But if they can live in peace with their unbelieving partner and prevent conflict then that is good. For Christians are to love their neighbours, including the unbelieving, and that includes an unbelieving partner. Indeed it must be recognised that there is a good chance that their influence might lead to their partner's conversion as well (1 Peter 3:1-2). Thus they too will enjoy God's saving power. The Christian seeks to spread peace and goodwill, although not at the expense of faithfulness to Christ, and to seek to win others to peace with God.
On the other hand it is not necessarily true that they would save their partners. How do they know? Thus they are not bound if the other partner leaves. The very act would reveal an obstinacy of heart against God.
‘Called us.' Here Paul is referring to God's activity in calling men to Himself. The verb is in continual use from now until 1 Corinthians 7:24. Whatever their state the Christian partners can see themselves as people whom God has called, people who are chosen of God and special in His eyes. Besides this their worldly station is irrelevant. They are now God's own, beloved people (Titus 2:14; 1 Peter 2:9).