In terms of why God allows it, I’m not sure — what I do know is that people have always been confused about Jesus, they were in the very beginning (read Mark 8 where Jesus asks the disciples “who do the crowds say I am?” and look at the range of answers!)
There is a very standard human response to God, which is to try to find ways to work towards our own salvation — through good works, piety, or even religion. (Look at Jesus’ dialogue with the rich young ruler for example). And some forms of Christianity have fallen into that trap — adding layers of religiosity on top of the gospel. Even when that’s well meaning, it can trap and entangle people, and prevent them seeing Jesus clearly.
With friends who are caught up in these traditions, I think we can gently call them back to the centrality of Christ — as clearly and graciously as we can.
A helpful book in all this is The Prodigal God by Tim Keller. Tim unpacks the parable of the lost sons in Luke 15, showing very well how the elder son in that story is intended by Jesus to challenge those who have made everything about legalism and empty religion.
You’ll also find lots of good resources (videos, articles) etc. at the Solas website: https://www.solas-cpc.org
Hope some of those thoughts help!