Addressed by Jesus (Matthew 19:3-9) and Paul (1 Corinthians 7). God hates divorce (Malachi 2:16) but permits it for sexual immorality and abandonment. Not all divorce is sinful; marriage designed for permanence. 55/100 Mixed.
Matthew 19:3-9 records Jesus going back to creation when asked about divorce: "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh. So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate." Jesus then acknowledged Moses' allowance of divorce while identifying it as a concession to human hardness of heart, not God's original design. Jesus permits divorce for "sexual immorality" (porneia) — an exception that acknowledges the marriage covenant has already been broken.
1 Corinthians 7:10-15 addresses mixed marriages — if the unbeliever leaves, "a believing man or woman is not bound in such circumstances." This "Pauline privilege" provides a second biblical exception: abandonment by an unbelieving spouse. Malachi 2:16: "The man who hates and divorces his wife... does violence to the one he should protect." God hates divorce — not because divorced people are uniquely sinful but because it represents the tearing apart of what God joined.
Not all divorce is equally sinful. Divorce in response to adultery or abandonment is permitted by Scripture. Divorce in cases of abuse is debated among Christians but many thoughtful pastors conclude the covenant has already been broken by severe ongoing abuse. The Christian church should be a community of grace for the divorced — maintaining the standard without crushing those who have experienced the reality. See our Is It a Sin? hub.
For a thorough biblical treatment: GotQuestions on divorce.
Questions about sin fall into two categories: things explicitly called sin in Scripture, and disputable matters (Romans 14-15) where Christians with different convictions should respect each other's consciences. Even when something isn't explicitly sinful: Does this practice reflect Christ's lordship over all of life (Colossians 3:17)? Is it beneficial — not just permissible? (1 Corinthians 10:23). Score: 55/100 Mixed.
See our Is It a Sin? hub. GotQuestions and the Gospel Coalition provide thorough evangelical analysis.