A few theologians, past and present, complain that this can't be true, since, if His intent is to save the world, then God has failed. And since God can't fail, therefore, God must not want to save the world. Theologians are second only to lawyers in coming up with things like this.
The answer is that God's intent is to make salvation a potential for everyone, and to actually save believers. This truth is illustrated by the bronze serpent on the pole (Numbers 21). God promised all Israel, without exception, that if they looked up at the bronze serpent, God would heal them of snake-bite. He promised He would heal any of them, and all of them.
God in His loving mercy invested into the serpent (symbolically) an objective power to heal. God's healing power was there, ready and able to act, even if no one had looked. Christ says that His death is just like that (John 3:14).
His death on the cross could save everyone, even though it actually saves believers only. If everyone without exception believed in Christ and His cross, His cross would save everyone without exception. That's how much redeeming power is resident in Christ's death. God is not a liar. God never promises things He hasn't provided.