The number five denotes divine grace in the Bible. In the fifth usage of "but God" we can see God's unmerited favor in the lives of men. God has called us to peace, and unless we realize the grace of God we will never manifest His peace.

1 Corinthians 7:15:
But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace.

  One will never manifest peace unless he knows that he lives by grace. The first words to the church directly after the salutation in each of the church epistles are "grace" and "peace." The Pastoral Epistles to Timothy and Titus similarly begin with "grace, mercy, and peace."

  Grace and mercy seem to be two sides of the same coin. Grace is unmerited favor. Mercy is withholding of merited judgment. Because of God's great grace and mercy, the believer has peace.

  Ephesians chapter five talks about the unique relationship of marriage. This unique relationship by proper arrangement and deliberate decision is only available to the faithful in Christ Jesus. When the attitude of husband or wife ever ceases to be faithful in Christ Jesus the unique relationship in marriage is no longer available. The faithful in Christ Jesus are those believers who faithfully walk according to the revelation of gospel of the great mystery revealed.

  I Corinthians 7: 1-15 deals with authority over the spouse's body, deprivation of sexual relationships, and breaking up of marriages. These are all interpersonal problems and result from not renewing one's mind to the Word and walking in love. None of these aspects are covered in Ephesians. Paul had to deal with these things in I Corinthians seven because I Corinthians is a reproof epistle. They were not walking according to the revelation of Romans. Ephesians is written to the faithful in Christ Jesus and assumes obedience to the revelation of Romans.

  Romans chapter four talks about Abraham's example. Verse sixteen says that the promise is of faith that it might be by grace. Chapter five continues:

Romans 5:1:
Therefore being justified by faith, we have peace with God through our Lord Jesus Christ:

  The believers already have peace. They have been justified. Therefore they have peace. They simply need to live and act peaceably.

Romans 5:2:
By whom also we have access by faith into this grace wherein we stand, and rejoice in hope of the glory of God.

  By Jesus Christ the believers have access into the grace wherein they stand, and rejoice in the hope of the glory of God. Because the believer is able to stand and look toward the hope of the return of Christ he can glory also in tribulation.

Romans 5:3-5:
And not only so, but we glory in tribulations also: knowing that tribulation worketh patience;
⁴ And patience, experience; and experience, hope:
⁵ And hope maketh not ashamed; because the love of God is shed abroad in our hearts by the Holy Ghost which is given unto us.

  God has called us to peace. However, we will never manifest peace and walk peaceably until we know that we stand in grace. God by His great mercy and grace has made salvation available. As a son of God the believer has peace with Him. If the believers individually have peace with God then they will have peace with each other. The manifestation of this peace will end the strife between husband and wife.

  The only way a marriage relationship has a chance to work is for both husband and wife to become faithful to walk in love and be likeminded on the Word. Then they can, by proper arrangement and deliberate decision, enjoy the unique relationship of marriage. However, should the unbeliever depart and by deliberate decision choose not to remain faithful to the Word and his marriage commitment, the remaining believer is not in bondage to keep a commitment that has become impossible for him or her to keep. Life is difficult enough when everyone pulls together. When someone severs their commitment to their spouse and leaves, the remaining spouse is released to pursue their relationship with God without the bondage and entanglement of past sin and mistakes. For God has called us to peace.