I do not know what prompted your question, but in an attempt to give an adequate answer, I will stress three things: First of all, divorce does involve or give evidence of sin, real sin, on the part of one or both of the marriage partners seeking the divorce. God’s revealed will and desire is that marriages be lifelong.
Second, the sin or sins normally connected with a divorce are certainly forgivable. Christ paid for all sin and God delights in pardoning all sinners. The personal enjoyment of forgiveness, of course, assumes that the sinner is brought to repentance and thus takes both the sin and the work of the Savior seriously. Only a lack of repentance and a willful despising of the gracious working of the Holy Spirit in unbelief fit the “unforgivable sin” category.
Third, some have been observed saying that, despite all this, divorce is sometimes treated as though it were unforgivable. This may stem in part from pastors and spiritual leaders striving to stress the seriousness of this sin coupled with its epidemic spread in our society. There are seldom any real winners, only losers, in a divorce. This may also stem from the popularity of what has been called “planned repentance” in divorce cases. This means that people willfully and wrongly seek a divorce with the conscious plan of “repenting” afterward, after the deed is done. Biblically speaking, that is not the pattern of true repentance. “Repentance” that is humanly planned and produced is not the real thing and is not accompanied by forgiveness.