Thats correct, they are not following Jesus if they are not doing what Jesus wanted them to do or if they are not teaching what the scriptures say. Jesus said that not many who claimed to follow him would be approved by him.
Matthew 7:21 Not everyone saying to me, Lord, Lord, will enter into the kingdom of the heavens, but the one doing the will of my Father who is in the heavens will. 22 Many will say to me in that day, Lord, Lord, did we not prophesy in your name, and expel demons in your name, and perform many powerful works in your name? 23 And yet then I will confess to them: I never knew YOU! Get away from me, YOU workers of lawlessness
There are many christians who are not following Jesus correctly, but it is not for other christians to judge them or force them to follow a particular set of beliefs....Jesus himself will judge them. We must individually do what the scriptures tell us to do and we must follow them...if a person is not doing that honestly, then they will be judged adversely for that error. That is what the scriptures say, not only my belief.
Paul and James gave this same council :
Romans 2:13 For the hearers of law are not the ones righteous before God, but the doers of law will be declared righteous.
James 1:22 However, become doers of the word, and not hearers only, deceiving yourselves with false reasoning.
So the fact that christianity is so divided and has so many different beliefs does not make christianity wrong... it makes those christians wrong. We still must follow Jesus and the bible. Just because many christians are not true followers, does not make the Word of God invalid and unworthy, nor does it change the fact that Jesus is the Messiah sent by God for all the world to follow him.
In the end, everyone will be judged by Gods Word. christian or not. And there are christians who do follow Christ according to the scriptures and they teach according to what is in the scriptures too. So there is a true Christian church... it is up to us individually to make an effort to find it. If we dont bother making that effort, then we will be held accountable for that decision.