Whenever we put a qualifier in front of the noun “Christian,” we might be inserting legalism. But we might not be. It depends on whether we perceive that qualifier as meritorious. Does it elevate us above other blood-bought Christians who don’t wave the banner of that same qualifier?
It is possible to be a “missional” Christian or a “radical” Christian or whatever, and that language is being used merely as a way of communicating something biblical that you want to call people to, something truly in Christ. But it is also possible — it all depends on internal factors, difficult to discern even in ourselves, much less in others — to use such qualifiers in a way that is truly legalistic.
Legalism is a serious accusation, as is obvious from Galatians. That makes me reluctant to use it in a targeted personal way, naming names. I could identify a specific man as a legalist only if (1) he makes an obvious theological blunder in writing, diminishing the finished work of Christ on the cross, adding something of his own to the empty hands of faith as the way of receiving that finished work, and he stands by his stated error even after appeals to reconsider, or if (2) I can have direct personal conversation with him and really press into what he means by what he says and I find out that, yes, he really is requiring more than the cross, received by mere faith, for peace with God. But without that clarification, legalism is an easy accusation to make, and a difficult one to prove. And any unprovable accusation is itself a wrong — a different kind of wrong, but still wrong.
It can get complicated, and quickly. Caution seems wise.