Does "one" have to mean a single ecclesiastical organization all under the same bureaucracy? Why can't it mean "one" in the sense of a general agreement on fundamentals - the Divinity of Christ, for instance - without necessarily everybody in the same organization?
I would imagine because Christ uses His relationship with the Father as an example. Christ is God just as the Father is God. They are the same. They are One.
The Body of Christ is not an organization. It transcends organizations. It is what it is, and one is either a member of the Body or one is not. The shepherd knows His own and they know Him -- even if, by accident of birth or brainwashing they nominally belong to other faiths. The Church exists everywhere. There is no salvation outside of it.
(And who will go to Heaven? That's not for us to say. I suspect that there will be a great many surprised faces to be seen on that first morning in Paradise -- and a good many on the first midnight in Hell, too. The best we sinners can do is "strive to enter" the reward prepared by God for His faithful.)
However, Christ did not leave us alone. He conferred the Holy Spirit upon His apostles and charged them to feed His flock -- to teach, lead, and comfort the faithful. To Peter He gave the keys to Heaven and Hell, the power to teach the Truth infallibly, and the authority to forgive sin in His name. The apostles in turn passed these on to their successors, who have carried the Keys for two thousand years without fail. We know these keepers of the Faith as bishops, and chief among them the Bishop of Rome, successor to Peter, who we call Pope. The Church consists of those united with Pope and Bishops -- even if they are united imperfectly with them.
Although we know that only the Church founded by Christ possesses the fullness of Truth, we Catholics call all those who profess the Creed by the name "Christian". Those outside of formal union with the Church are our separated bretheren. Some of our separated brtheren are nearer or further from the fullness of Truth according to which self-proclaimed "pope" they follow -- Luther, Calvin, Zwingli, Benny Hinn, et al -- but all who acknowledge the Creed are members of the Body of Christ nonetheless.
In this sense, all Christians are one, part of the singular, ressurrected Body of Christ. However, until the Age of the Self-Styled Popes comes to a close, the Passion of Christ will continue, the Body of Christ will writhe under the whip of this world -- wounded, flayed, suffering. To end its torment we must all work and pray, suffer and sing, and most of all submit ourselves, our wisdom, and our own pride to the authority of Christ established by Him on Earth: Peter, the Rock upon which the Church is founded.