Question: "What should be the response of a Christian whose spouse has had an affair?"
Answer: Infidelity is a very difficult and painful situation. It involves all the emotions, and, for the Christian, can stretch faith almost to the breaking point. The best thing to do is to "Turn all your worries over to Him. He cares about you" (1 Peter 5:7). Go to Him for daily comfort, wisdom, and direction on a daily basis. God can help us through the deepest of trials. Adultery is always wrong.
"God will judge the person who commits adultery" (Hebrews 13:4). The injured party should rest in the truth that God is the avenger. The person who has been sinned against does not have to fret over getting even. God will do a much better job of that. When we are injured, we need to commit this sin of adultery to the One who knows every detail and will deal with it appropriately.
"FORGIVE people when they sin against you. If you do, your Father who is in heaven will also forgive you. But if you do not forgive people their sins, your Father will not forgive your sins" (Matthew 6:14, 15). This may seem impossible, but with God's grace, the victim can make it an act of the will in obedience to God's Word. To harbor bitterness will affect attitudes, emotions, and the desire to obey God, and it will negatively affect everyday decisions. This does not mean that the injured spouse is not going to suffer the effects of deep hurt. But the grace of God will greatly minister to his/her needs. The alternative is an unforgiving heart that will soon be racked with vengeful thoughts, anger, wrath, etc.
BE FORGIVEN. "But God is faithful and fair. If we admit that we have sinned, He will forgive us our sins. He will forgive every wrong thing we have done. He will make us pure" (1 John 1:9). Both parties should ask God to help them see how each may have contributed to this whole situation and be released from the weight of guilt before God. From that point on, there will be freedom to seek His counsel and guidance. His Holy Spirit will enable them to do what they could not do on their own. "I can do all things through Christ, who strengthens me" (Philippians 4:13).
Then as God leads, forgiveness and reconciliation can follow. Even if this takes time, every effort must be made forgive and reconcile. (See Matthew 5:23-24.) As to whether to stay or to leave, "Anyone who divorces his wife and gets married to another woman commits adultery. A man may divorce his wife only if she has not been faithful to him" (Matthew 19:9). But while the innocent party may have grounds for divorce, God’s preference is forgiveness and reconciliation.
The Lord says, "I hate divorce!" (Malachi 2:16) It is far better to try to resolve the issues causing the problems, especially if there are children involved. Godly counsel is helpful from someone who uses God's Word—not Freudian psychology—as the basis for their advice. Prayer for guidance in all thoughts, words, actions, and decisions is crucial.