Many denominations could suggest that Christianity is rooted in many different cultures and that the personalities and idiosyncrasies of each of these cultures causes them to worship in different manners and forms. A church in Fiji would probably worship and emphasis certain practices differently to say a church in England, or in underground China.
Denominations may also suggest that many Christians have thought extremely hard about their faith and have come to different conclusions about certain doctrinal issues. The Christian could argue that disagreement in interpretation does not necessarily show that a proposition is false; rather it shows that they simply disagree on that proposition.
Thirdly, according to biblical theology humans are fallible creatures and one could expect mistakes in interpretation to crop up. However, it wouldn’t follow that because Christians misinterpret doctrine that it means it is false.
Fourthly, the Christian could note that there are a number of beliefs that all Christians must affirm in order to be a Christian irrespective of disagreement. For instance, belief in God’s existence, the deity of Christ, sin, the fall, and salvation, among other essentials would need to be embraced by the consistent Christian. One cannot be a Christian if they deny the existence of God, or the hypostatic union of Jesus. So whether one is a Baptist, Catholic, Methodist, or Protestant these things are affirmed unanimously, only on other secondary beliefs do they disagree with each other.