I have been reading John Piper's (2006) book "What Jesus Demands of the World". This is an excellent read, presenting Jesus' authoritative commands to us and urging us to God-glorifying obedience. The book consists of 50 demands, starting with #1 You must be born again and ending with #50 Make disciples of all nations. Any Christian will profit greatly from reflecting upon these.
Several demands concern marriage: #40 (What God has joined together let no man separate, for marriage mirrors God's covenant with us), #41 (Whoever divorces and marries another commits adultery), and #42 (One man, one woman, by grace, till death).
Piper argues that, according to Jesus, marriage is a great work of God and a sacred covenant breakable only by death (p.301). Jesus set a higher standard for marital faithfulness than Moses; he did not affirm the permission of Deuteronomy 24 (p. 307). According to Piper, the "porneia" in the exception clause of Matt.19:9 refers to sexual immorality before the consummation of marriage. Hence, once married, there are no valid grounds for divorce, let alone re-marriage. Marriage is for life.
Reacting to such high standards, the disciples replied, "If such is the case of a man with his wife, it is better not to marry" (Matt.19:10) for, as Piper comments, "if there is no back door to marriage, it is better not to walk through the front door" (p.317).
I believe that Piper makes a convincing biblical case against divorce and re-marriage.
Less convincing to me is Piper's practical advice on how to deal with actual cases of divorce and re-marriage. Piper contends that the couple should confess that the act of re-marriage was a sin, and seek forgiveness, but to continue in the second marriage. In effect, however, this allows for divorce and re-marriage for any reason--as long as one later repents. This seems to undermine Piper's previous principial arguments for life-long commitment: in practise there is, after all, an easily-accessible back door out of any marriage.
Piper lists several grounds. The most pertinent is his contention that covenant keeping is crucial to Jesus; therefore, even though the current covenant is adulterous in the making, it is real and should be kept (p.321).
This rationale seems flawed. Since covenant keeping is crucial to Jesus, and since Jesus teaches that marriage is for life, surely this underscores the necessity of keeping the vows of one's first marriage. Further, if re-marriage constitutes adultery then the vows involved in re-marriage amount to no more than sinful vows to commit adultery. Sinful vows are to be repented of, rather than kept. They are certainly not to be elevated so as to trump one's earlier, God-honouring, "till death do us part" marital vows.
Of course, pastorally, it is much easier to follow Piper's pragmatic advice then to void the re-marriage. Nevertheless, if, as Piper maintains, marriage is indeed a sacred covenant breakable only by death, then it would seem that re-marriage has no more validity than bigamy. If that be the case, how can the church possibly acknowledge such re-marriage as legitimate?