What should be the response of a Christian whose spouse has had an affair?

Infidelity creates a very difficult and painful situation, one that
involves all the emotions, and, for the Christian, can stretch faith
almost to the breaking point. The best thing to do is "turn all your
worries over to Him. He cares about you" (1 Peter 5:7). Go to the Lord
for 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 betrayed, we need to commit the pain to the One who
knows every detail and will deal with it appropriately.

FORGIVE. "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, and it will take time,
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. Refusal to forgive is more detrimental to the
offended party than it is to the offender. This does not mean that the
betrayed spouse is not going to suffer the effects of deep hurt.
Forgiveness also does not make the offense "okay." Forgiveness is
about receiving God's grace and trusting Him to redeem everything in
our lives. It is appropriate to engage the anger and hurt caused by
infidelity. Expressing these emotions to God can be a first step
toward true forgiveness. Forgiveness is an act of the will, but it
also must be heartfelt. Giving our emotions and needs over to God
allows Him to minister to our hearts so that we can let go of the
offense and forgive as we have been forgiven.

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 the 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. No
matter how long it takes, every effort must be made to 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). 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 from a Christian counselor or
pastor is helpful. Prayer for guidance in all thoughts, words,
actions, and decisions is crucial.