Yes, we should forgive all. We see verses like,

"Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you."-Ephesians 4:32

"For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses." -Mathew 6:14-15

And we are called to repent and be forgiven (by God),

"If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. " -1 John 1:9

"Repent therefore, and turn back, that your sins may be blotted out" -Acts 3:19

From verses like that we see that forgiveness from God only happens by repentance on our part. The same is true for us forgiving our brother, if your spouse cheats on you, and asks for forgiveness then you should give it. If your son's murderer asks for forgiveness, then give it. But don't confuse forgiveness for injustice

"Learn to do good; seek justice, correct oppression; bring justice to the fatherless, plead the widow's cause." -Isaiah 1:17

"He has told you, O man, what is good; and what does the Lord require of you but to do justice, and to love kindness, and to walk humbly with your God?" -Micah 6:8

If you pretend like your son never got murdered, you are doing an injustice to him. In that case you should take the murderer to court, which would be doing justice. We see that God, even though he forgave our sins, poured out his wrath on Jesus, he didn't just sweep it under the rug and forget about it.

"He himself bore our sins in his body on the tree, that we might die to sin and live to righteousness. By his wounds you have been healed." -1 Peter 2:24

"Since, therefore, we have now been justified by his blood, much more shall we be saved by him from the wrath of God." -Romans 5:9

To sum it up: Yes, we should forgive. Yet we must also remember that forgiveness isn't "Going with the flow" or forgetting about wrongs done to others.

I hope I answered your question, sometimes my thoughts can come out wordy and jumbled so feel free to ask any questions you have :)

"The Lord bless you and keep you; the Lord make his face to shine upon you and be gracious to you; the Lord lift up his countenance upon you and give you peace." -Numbers 7:24-26

Should we forgive everything?

Yes, but only when forgiveness is deserved.

Why should a Christian forgive their girlfriend/boyfriend or spouse if she has betrayed their trust and cheated on them with another man?

Forgiveness isn't the same as forgetting or suppressing memory.

Suppose someone borrows an item from you and returns it broken, with no real apology or offer of compensation.

You can make a choice that will affect the rest of your life:

• Resent, or even hate, that person, perhaps with ongoing fantasies of revenge and punishment.
• Make a mental note to never lend anything to this person again, and continue your relationship with them.

The significant difference between the two attitudes is the effect it will have on you, yourself, not on them.

With one choice, you will be carrying a negative feeling with you for the rest of your life; sometimes subconsciously and sometimes openly; sometimes mildly and sometimes to the point of obsession and PTSD. This will hurt you, your family, and your friends and colleagues. The person that offended you will at best notice that there is something wrong with you, but will very unlikely ever realize their own involvement. Your emotions and desires will be totally wasted on them; it's only yourself that will suffer.

With the second choice you will still be your normal self, will still get on with your life, and will have an additional piece of knowledge that will help prevent you from making the same mistake again.

Now suppose someone borrows an item from you and returns it broken, but apologizes and offers to pay for repairs or replacement. Perhaps they even admit that it broke because they used it improperly, and now know not to do that again.

You might:

• Accept the money.
• Tell them thanks for the offer, but it was old and you were expecting it to break soon anyway.

But whatever you choose there shouldn't be any continuing hard feelings about the incident, you have forgiven them, the incident is done with.

The difference between these two incidents is the attitude of the offending person. One person didn't care that they had damaged your property, the other not only felt bad about it and knew that they owed you, but indicated that they would change their behaviour.

In the first incident above, the other person hasn't admitted their fault. You deal with your own feelings, either poisoning yourself, or accepting that some people are just like that. Either way, you aren't really forgiving them.

And that's okay, they don't deserve forgiveness. But neither do they deserve punishment, hatred, or your dreams of vengeance. You must forgive them only in the sense of not resenting them or holding other bad feelings about them.

Yes, some people might actually say they are sorry, but what they really regret is only that it happened or that they got caught; their own part in the event is not worth their consideration. There is no repentance, no desire to change their behaviour and character.

But in the second incident, the person has offered compensation and has shown regret (knowing they did something against you) and repentance (willingly changing their ways).

God is love and forgiveness, but should we forgive everything?

From a purely secular point of view, resentment and hate are very bad choices for one's mental health, while forgiveness is good for everyone.

From a Christian point of view, the same can be said, but with much deeper significance.

“Take heed to yourselves. If your brother sins against you, rebuke him; and if he repents, forgive him. And if he sins against you seven times in a day, and seven times in a day returns to you, saying, ‘I repent,’ you shall forgive him.” — Luke 17:3–4

Notice the dependence upon repentance. It's not possible to forgive someone that doesn't know they need forgiveness.

Offering compensation and showing regret and repentance is exactly what God expects of Christians: recognizing when one has sinned and as a result positively changing one's character.

Consider Jesus's model prayer in Luke 11 (NLT): "forgive us our sins, as we forgive those who sin against us". Christians ask God to forgive their sins, but they can't expect God to be any more forgiving toward them than they are to other people.

Forgiveness is the core of Christianity. Not only can one expect forgiveness from God, who is willing to pay all debts, but one must build a character that doesn't harbour grudges, resentments, and other negative emotions. Building that perfect character, the image of God, is the purpose of life.

It's not possible to love God and at the same time hate any person (including oneself).

Why should one forgive their son's murderer?

As with your other two example questions, the answer to "why one should forgive" is because the one doing the forgiving will benefit from it far more than the one that receives the forgiveness. Once one truly forgives, the negative thoughts that weigh one down throughout life are eliminated. One cannot help but feel better.

One can only imagine how much it pleases God himself when someone changes their ways and receives his forgiveness.

You might want to read The Power of Forgiveness, which provides a powerful report of how one community reacted to an insane gunman's slaughter of their school children. Rather than the destructive path of hatred and self-pity that most people would follow, they instead chose understanding and love.

Lord, how oft shall my brother sin against me, and I forgive him? till seven times? (Matthew 18:21)

Meaning of "Seventy times Seven"

Jesus taught "forgive seventy times seven." This is intentional.

There are seventy original nations of the world.

There are seven days in a week.

Forgive "Seventy times seven" = Forgive all people, at all times.

Why Forgive?

Our forgiveness costs us nothing. It is merely acceptance of the fact that the other person will be held accountable individually before the Lord (not by us; God is their Judge), and is free to repent if he so chooses. We will certainly not hold him back, nor will we hold ourselves back by keeping grudges. Holding a grudge damns ourselves, or in other words, it halts our progress because we choose to believe something that is untrue about ourselves, others, and about God's plan of mercy. Holding a grudge denies the Atonement of Christ, and is contrary to His plan. Forgiving does not mean forgetting. We are all privileged to act wisely, while not passing final judgment on anyone.

What really matters is the Lord's forgiveness. All the people in the world could forgive you but that isn't worth two celestial cents. If He forgives you then you have gained heaven; whether or not others choose to forgive is only them choosing for themselves whether or not they accept the Atonement of Christ and the Lord's plan of individual accountability enough to move forward with their own lives without demanding eternal punishment. Choosing to let go of a grudge is the decision to trust that God will judge righteously with or without our input; He knows all things and is perfectly just; best to leave judgment alone with Him.

God will not forgive everyone, because not all people will choose to repent. He does abundantly pardon all those who do repent and seek forgiveness. Whether or not He pardons someone is not our business. Our business is to love mercy, and keep God's commandments, and help our brothers and sisters to find repentance. He who finds repentance, finds mercy.

"Blessed are the merciful, for they shall obtain mercy".

Therefore, to obtain mercy, have mercy on others. This does not mean we must enable bad behaviors. Justice must be upheld. The two are in no contradiction: When we act justly, mercy becomes possible. When we are unjust, we cannot have mercy, because mercy cannot take the place of justice. Both must be satisfied, and this is the work of the Redeemer and the Mediator. Our work is to bring the souls of men to this point, of applying His redeeming grace through faith and repentance, and so forth.

Failure to forgive is an obstruction to the great work of redemption, the work that is shared by God and man.

Yes we should. But beware of some traps that a simplistic understanding of "forgiveness" may bring.

Other answers have laid down the theoretical theological grounds for being prepared to forgive all, which I agree with in principle. However this can sometimes lead to an unrealistic approach to relationship. and forgiveness. Here are some principles that will keep us out of the traps that universal forgiveness can get us into, allowing us to be "wise as serpents" as well as "innocent as doves".

Forgive does not mean forget

If someone steals from you, and ask for forgiveness you might forgive them. But that does not mean you have to trust them with your money a second time. You are entirely justified in taking precautions to protect yourself. Don't give them your house keys or your bank card.

This is particularly applicable to abused women, where the abuser often shows great remorse after the abuse, but then repeats the behaviour in a predictable pattern. The victims can remain trapped in the abusive relationship, and churches sometimes encourage this by telling the victim they should "forgive" the abuser. It is possible to forgive the abuser and still get out of the relationship.

Beware Cheap Forgiveness

It's easy to say the words "I forgive you" to someone, especially if you think it is the 'right thing to do'. It is much less easy to come emotionally and spiritually to a place where you have actually forgiven them inside yourself for something serious. Sometimes doing the "quick and cheap" statement of forgiveness will hinder the journey to actual forgiveness, which takes time and emotional effort. It is that real, internal forgiveness that brings the emotional and spiritual benefits outlined in the other answers. With "quick and cheap" forgiveness you can still end up hurt and resentful many years in the future. You might consider consulting a grief counsellor. My wife was one, which is why I'm writing this. (I'm not advertizing, she's retired.)

Forgiveness does not mean absence of consequences

A truly repentant person will seek to redress the hurt they have caused, and to take real steps to prevent it happening again. They will also accept the consequences. Forgiveness doesn't mean you should ask the police to release the person who stole from you. You may choose to break up with your cheating boyfriend, and that doesn't mean you haven't forgiven them.

Short answer

Yes, we should forgive everyone. It's a command that we can do with the help of the Holy Spirit. Following God's example who forgave Israel FIRST, we need to forgive our enemies in our heart FIRST, while waiting for their repentance which may never come. In the long run, forgiving is beneficial for us.

The command

For Christians, forgiving our enemies is clearly commanded by Jesus (Matt 5:43-48):

⁴³ “You have heard the law that says, ‘Love your neighbor’ and hate your enemy. ⁴⁴ But I say, love your enemies! Pray for those who persecute you! ⁴⁵ In that way, you will be acting as true children of your Father in heaven. For he gives his sunlight to both the evil and the good, and he sends rain on the just and the unjust alike. ⁴⁶ If you love only those who love you, what reward is there for that? Even corrupt tax collectors do that much. ⁴⁷ If you are kind only to your friends, how are you different from anyone else? Even pagans do that. ⁴⁸ But you are to be perfect, even as your Father in heaven is perfect.

especially if the person who sins against us is a believer (Matt 18:21-22):

²¹ Then Peter came to him and asked, “Lord, how often should I forgive someone who sins against me? Seven times?” ²² “No, not seven times,” Jesus replied, “but seventy times seven!

Forgiving those who wronged us is frequently linked to our Father's forgiving our sins as well (Lord's prayer Matt 6:12 and the immediate verses that follow: Matt 6:14-15):

¹² and forgive us our sins, as we have forgiven those who sin against us. ¹³ And don’t let us yield to temptation, but rescue us from the evil one. ¹⁴ “If you forgive those who sin against you, your heavenly Father will forgive you. ¹⁵ But if you refuse to forgive others, your Father will not forgive your sins.

The role models

But God Himself knows how hard and sacrificial it is to forgive those who wronged us. He did what He preached! God himself (in the OT) forgave Israel who worshipped idols (an adultery in the eyes of the Lord) and who murdered the righteous innocents (which God took to heart as murder of His own sons and daughters). We read how God overlooked the offense in order to bring life to those who repent. Repentance and restitution is necessary, but God forgive us first, by making the first move of reconciliation by sending His Son Jesus.

We can take comfort that Jesus himself has also set an example for us (Luke 23:34), and so has Stephen (Acts 7:60), James, and countless martyrs and saints, to show that it is possible when one has faith that Jesus has already forgiven us.

The benefits of forgiving (or the danger of not to forgive)

But more than possible, many faithful in the church testified (and continue to testify today, as we read in many biographies) how forgiving is good for our soul and body as well.

On the other hand, grudge, resentment, and bitterness

• are incompatible with joy and peace, and can in fact lead us to curse our enemies, wishing them harm (the antithesis of love)
• can cause us to commit crimes of revenge, which if we are punished by the law will not only harm us but those who love us!
• will tax our energy, memory, and time which could be better used for recovering our loss by making investments for the future (new relationship, another child)
• can provide Satan for foothold to harass us
• limit our soul's freedom for love
• can develop bodily sickness as well

The "sweet justice" of revenge is simply not worth pursuing in the long run compared to the blessings we will receive when walking in the right path.

Addressing the loss and "forgiving" God

There are two very difficult obstacles to overcome in order to forgive, especially in the 2nd example cited in the OP (murder of son):

1. The hurt stays with us possibly for the rest of our lives for what someone does to us in an instant, especially if the person doesn't even repent or doesn't receive appropriate punishment through the civil justice system.

2. Where is God and how can He allow this to happen? This is especially if we have been a good Christian with healthy relationship with God. Christianity teaches that God is supposed to be all powerful, all knowing, and all loving. We are not the wicked, why did He allow this to happen to His beloved children?

Unfortunately, the Bible doesn't provide an answer (God refused to answer Job). God only says that He weeps with us (John 11:35), cares for us (1 Peter 5:7), will wipe away all tears (Rev 21:4, Isa 25:8), and calls us "blessed" (Matt 5:4: "Blessed are those who mourn, for they shall be comforted").

But where is the present comfort? Is God a sadist? Is He really good? It is said that God allows evil so good may come out of it, but what is it? In the betrayal of spouse case, at least one spouse is happy although at our cost. Maybe we learn something to improve ourselves. Maybe it's "good riddance" (better early revelation of the spouse's character than later). But what good comes out of the death of a son?

Several comforts have been proposed:

• God wants to remind us that all created things and creaturely relationships are supposed to be enjoyed IN God, not apart from God. It's very easy to cherish someone / something but forget the ultimate giver. Christians' ultimate desires need to be God alone so the creaturely loves don't turn into an idol.

• It's for our character development. It's painful, but necessary for us.

• It's to prepare us for a future ministry (example: Dr. Craig Keener's Impossible Love story after his first wife left him)

• It's a test of faith (like Job).

• It's part of God's plan so we can sympathize and offer comfort to others in the future.

• God plans to give us better blessings.

But none of the above is guaranteed since only God knows what we need and what His plan for us is. At the very least though, we can rely on God's promise that in heaven God will wipe away every tear (Rev 21:4).

Conclusion

Yes, at first glance, our enemies don't deserve our forgiveness, especially if they don't repent. Yes, it's heartless to demand victims who still suffer to forgive the very agents causing them great emotional harm. Forgiveness has to come from within, and only time and supernatural love makes it possible.

Forgiving does NOT mean we become friends with them (especially if they don't repent), or become a doormat (allowing them to hurt us again and again), but simply to release the matters of revenge and judgment to God (which He promises to do on Judgment Day). Loving enemies here means that we give them the opportunity for God to reform their lives just as we have given ourselves opportunity for the same when we became Christian (see my answer to another question Are we to love people whom we do not trust?). Therefore, forgiveness is an expression of loving others as we love ourselves.

But we are not alone! Jesus (who is the perfect human being) has done it when he was crucified for sins he did not commit. Through the new life given to us, the Holy Spirit empowers us to do it too since love is one fruit of the spirit (Gal 5:22-23):

²² But the Holy Spirit produces this kind of fruit in our lives: love, joy, peace, patience, kindness, goodness, faithfulness, ²³ gentleness, and self-control. There is no law against these things!

Therefore, if we value joy, peace, and forgiveness for our own sins from God, we can implore God to infuse us with faith and love so we can forgive our enemies as well.