A Christian CANNOT believe whatever he wants.
There are set rules and beliefs which must be accepted if you want to be called a Christian.
A buddhist's beliefs, for instance, are not Christian.
It doesn't seem to me that this is something that even needs to be said.
Thinking and believing are different.
Thinking involves thoughts and ideas
Believing involves thinking, which engages action.
Christians have all sorts of crazy ideas involved in their spirituality; some orthodox, some heretical and others, undetermined. Jesus calls us to love. If we get too caught up in perfecting our thoughts on doctrine, morality and spirituality we become prideful. Best to focus on working with God to perfect our love.
Indeed, our sanctification happens when our thoughts, emotions, and will are on in agreement and compel us to love and receive love perfectly. But, there is no order in which this happens - sometimes unorthodox thoughts or emotions (intentions, motivations) lag behind our ability to love; sometimes our love remains self centered even when thoughts are orthodox.
We are perfected on the narrow path even when we run askew.