First of all, Christ addresses the issue of people who do things in Christ's name but who do not follow exactly with the Apostles in Luke 9:49-50:
49 John answered, “Master, we saw someone casting out demons in your name, and we tried to stop him, because he does not follow with us.” 50 But Jesus said to him, “Do not stop him, for the one who is not against you is for you.”
So, as long as I believe in Jesus and accept him as my Savior, I am allowed to form my own opinions and still be considered a Christian by those who do not share my views. For instance I do not believe that Jesus is equal to the Father, nor is he G-d, but the Son of G-d. Are you ok with that? I'm still considered a Christian in your eyes, are I not? I don't believe in a Hellfire, that Hell is not a fiery place of torment, but that Hell (Hades in Greek) is man's grave, or "the pit" and that people will be resurrected from "Hell". Even though I believe this, I still have Jesus favor and I am considered a "true" Christian in your eyes, correct?
My point is this: If we apply Luke 9:49-50, I can have my beliefs, and you can have yours, and we disagree on many fundamental issues, but as long as "we are for Christ", we are both true Christians, right?
The "thousands of denominations" is a really old Roman Catholic canard as well. First, lots of them are liberal non-churches who are only nominal Christians (UCC, PC(USA), EPCUSA, etc.).
Wait, you are judging here, and Jesus was emphatic when he said "Stop judging!"
Who are you to judge whether these different denominations are "liberal non-churches", or only "nominal Christians"? Maybe they don't believe as you do, but remember, Jesus said, if they "are not against you they are for you". We have to be careful not judge if we do not want lose G-d's favor.
Secondly, lots of the denominations are simply national divisions, such as the Presbyterian Church in America as opposed to the Free Church of Scotland or the Presbyterian Church in Australia. Also, within a country divisions within a broader denomination (such as the Orthodox Presbyterian Church as opposed to the Presbyterian Church in America) is because the liberal domination of the primary denomination in a country. In this country, the PC(USA) has become more and more liberal, causing Evangelicals to split with it over the course of almost a century now, and that continues to this day. In these cases, the denominations often have agreements that basically make them one denomination for all purposes besides church government.
So you are saying that these different denominations are all the same religion, they are just in different locations? You are saying that they are all fitly united in Christ "in the same mind and same line of thought"?
In the end, there are only really around eight denominations, which all recognize each others' claims to being Christian and do not deny the others' Christianity: Baptist, Presbyterian, Reformed/Low Church Anglican, Continental Reformed (arguably under the Presbyterian banner or visa versa), Lutheran, Anabaptist, Congregationalist, and broader Evangelical "non-denominational" churches. Outside of this, the divisions are either significant enough to deny the others' Christianity, or not worthy of being labeled a separate denomination.
Religions recognizing each others' claims to be Christian does not mean they are united and in agreement. In fact, it is just the opposite. The Black Baptist go to their own church in the South because the white Baptist do not want any Blacks in their church. The Baptist Church is only one example, but the different Baptist churches are very divided in their beliefs, and yet they can't all be right. They are also not following Paul's admonition to be "fitly united in the mind and same line of thought".
What religion do you belong to? Do you consider your church to be the true church?