Hebrews 3:7-13

Take a look at Psalm 95, the source of this quote.
"Come, let us sing with joy to the Lord;
Let us shout aloud to the Rock of our salvation." (Psalm 95:1)

This song of joyful praise ends,
"Do not harden your hearts
as you did in the rebellion...
So I declared on oath in my anger,
'They shall never enter My rest.'" (v 8,11)

The author of Hebrews concludes,
"See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called today, so that none of you may be hardened by sin's deceitfulness." (Hebrews 3:12-13)

What makes God angry?
The message of Psalm 95 is the message of this passage: Praise God! Worship Him joyfully, for He is worthy of praise! Worship Him, because He has saved you and given you good things and taken care of you! Worship Him, because He takes care of you daily!
For if we do not; if we, rather, choose to quarrel with God and reject Him as God, then we will not know His peace.
This is a scary passage, and many preachers will play upon the fear it invokes. Don't be excited into rash judgments or false beliefs just because a silver-tongued preacher breathes brimstone at you and inundates you with hellfire clippings of bible. Be convicted by the word of God, and let it guide you into truth.
However, don't gloss this verse over in your mind, either--it is scary, and it's intended to be. This is David's warning to God's people that they need to worship God and to keep Him in their minds.

But what was it that made God angry, and why does it apply to this message? David's reference comes from Exodus 17, and you need to turn there if you want to get to the bottom of the matter.
The Israelites are thirsty because they have no water, which is understandable. They turn to Moses and say, "Why'd God bring us out here without any water? We were better off without Him!" This, after they'd seen Him provide for them... The previous chapter, God first gave them mana and quail. They were eating the mana God had provided for them, but they were accusing Him of not taking care of them.
That's just wrong. No? You may be able to imagine God's anger at this.

God takes care of us. He provides for us, even when things are hard. He saves us, even when we don't deserve it. What's our response to all this? What should be our response?

Hebrews 3 is about cause and effect. God takes care of us without grumbling, so we should praise Him joyfully. It's only natural! When the people of Israel did the unnatural thing and grumbled against God and rejected Him after His care for them, God did the natural thing and got (rightfully!) angry at them.
Did He stop taking care of them? Did He disown them as His people?
No; He forgave them--but He didn't let them go unpunished. He gave their reward to their children, instead of letting them enjoy it.

We receive care, just as did the people of Israel. We receive God's love, and we receive the same salvation from evil. Our response is the same: natural love, natural praise. If, instead, we offer unnatural hatred and rejection, God's natural (and rightful) response is anger.
Will He disown us? Will He not forgive us? Perhaps, but how will God reward us if we reject His blessings as a curse? How can we receive blessings from God if we believe that?

Make no mistake, rest here is a reward--and that reward is for the faithful.
May we be faithful today as we receive and enjoy God's blessings.