What is the Biblical basis for "God's already forgiven you" teaching?

If Osteen and Prince were assuming an unsaved audience when they uttered this phrase, then it makes no sense. Forgiveness comes from confession of sin to God or Christ, and the unsaved person by definition does not believe in Jesus.

If they were assuming a saved audience, then the phrase "God has already forgiven you. All you have to do is accept God's free gift of salvation." still doesn't make sense because a saved person by definition has accepted God's free gift already.

Perhaps they were addressing someone who has been forgiven of a great wrong done toward a Christian and the Christian has forgiven that person. That is the only scenario in which I can see the passage making sense. "You have been forgiven by the one they wronged and feel peace about it; now why not admit your wrong to God and find a greater peace?"

It's hard to figure out the meaning without getting the context of the speech.

Where your confusion seems to lie is in the fact that salvation (forgiveness) is a gift from God. This is what Christians believe.

Jesus died on the Cross so that our sins might be forgiven:

Matthew 20:28 KJV

Even as the Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many.

The reason for this is rather simple: salvation is a gift from God due to his enormous grace.

God sacrificed his son on the cross so that we might receive his gift of salvation, but just as any other gift it must be received by the person to whom it is presented.

For instance if you never open your Christmas gifts, they are still gifts, but they do not do you any good until you do open them and begin to use them. And if you never do it is not the fault of the person who gave the gift.

So it is with salvation. It is there just waiting -- for acceptance, and even beyond acceptance -- to be used by the receiver. The two go hand in hand.

When Jesus died on the cross, God heaped on him punishment for all the sins of the World, but it must be remembered that as terrible as it sounds that too was a gift (albeit a miserable gift), and as with all gifts Jesus had to accept them.

Colossians 2:13 KJV

And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;

We must also remember that a gift not only has to be received, but it must also be given. And remember that those are our sins, they do not belong to either God or Jesus, therefore it is we who must make a gift of them to Jesus so that he too can make use of them to justify his dying on the cross. Since he has to take the punishment, we should at least make it worth his sacrifice.

The next step would logically be to do that, but the question is how can we?

Well that's a good question and I'm glad you asked it!

1st John 1:9 KJV

If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.

Here is how it is done:

1. We must admit that those sins belong to us. (that is confession)

2. We must ask God to allow Jesus' death on the cross to be the remission of our sins (ask for forgiveness).

God will then turn to Jesus (who is at his right hand) and ask if he will accept your sin. (Of course Jesus will say yes since he already suffered torture in your name.)

At that time your sins will be taken from you and sacrificed on the cross with Jesus. That is forgiveness.

Believe then that your sins have been paid for in full, and you have been born again, not of the flesh but of the Spirit, and you have become worthy to enter Heaven since you are now sinless in God's eyes, and have become perfect as Jesus was perfect.

The key word here is believe.

1st John 1:12 KJV

But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:

As I said before a gift is only a gift not only when it is opened, but it must also be used.

Suppose you are given a car, and all you do with it is set it up and look at it.

It is still a gift and it is yours but until you start to drive it you are not making use of your gift.

There are two ways in which you can use your gift of Salvation, you can bask in it, which God wants you to do.

John 10:10 KJV

The thief cometh not, but for to steal, and to kill, and to destroy: I am come that they might have life, and that they might have it more abundantly.

I like the "more abundantly" part!

And you can also share it. (Which is what I am doing now.)

Here are some other Scriptures which help to explain.

Mark 4:12 KJV

That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them.

Matthew 9:6 KJV

But that ye may know that the Son of man hath power on earth to forgive sins, (then saith he to the sick of the palsy,) Arise, take up thy bed, and go unto thine house.

Ephesians 4:32 KJV

And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you.

Grace is the forgiving of sin:

Mark 2:5 When Jesus saw their faith, he said unto the sick of the palsy, Son, thy sins be forgiven thee.

God’s grace is a gift:

Ephesians 2:8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:

“Faith” enables “grace”:

Romans 4:16 …it is of faith, that it might be by grace…

God provides “faith”:

Romans 12:3 …as God hath dealt to every man the measure of faith.

Grace while ratified upon the cross is eternal:

Genesis 6:8 ..Noah found grace in the eyes of the LORD.

A gift by nature is given in advance, out of love.
A wage is given after ones works are measured.

Grace is God's forgiving, because it is a gift it has to be fore-given!

From a Calvinist perspective, any teaching which makes God contingent on man in any way negates the supremacy of God. Put another way, who is man to say anything to God? Any choice which man makes for God is the result of an election that occurred ante praevisa merita, and is thus a decision man made long before any person "decided" he or she wanted forgiveness. In such a scheme, to say that God has "already" forgiven you makes logical sense, because God, who exists before and outside of time, would have already made the decision. Any scripture backing predestination thus applies, such as Ephesians 1:4:

even as he chose us in him before the foundation of the world, that we should be holy and blameless before him (ESV)

From an Arminian perspective, of course, free will is paramount, and the above argument would be nonsensical at its core. As such, for Methodists and the like, the timing of God's forgiveness could be a question - but Osteen in particular isn't Methodist. But, a Methodist would state the obvious - if Christ's death isn't sufficient for future sins, what is? Hebrews is clear that:

so also Christ died once for all time as a sacrifice to take away the sins of many people. He will come again, not to deal with our sins, but to bring salvation to all who are eagerly waiting for him. (Hebrews 9:28)

As such, even from the polar opposite soteriology, the timing of Christ's forgiveness would predate any individual's decision.

Finally, it should be noted that Joel Osteen has never been to seminary, nor does he profess to be "much of a theologian", so don't expect too strong of a case if you're basing it on him.