The Lord's Supper.
Mysterious, or straightforward and simple? It seems to me that we've bought into the modern way of things to a great degree on these things. (In what's written compared with the past, in attitudes I detect in me, and in what's practised by churches.)
We don't like mystery in theology, in theory. No thanks - we'll have neat systems, simplified and based on us. 'Not me', you say? For example, when was the last time you heard someone leading a service refer to a sacrament as 'a sign and seal of the covenant of grace' (or explain that concept in different words)? No, that's far too complicated for today - and why, it would imply that God does something in relation to Christian baptism and the Lord's Supper (and then we'd have to think how and why and so on)! We'll call it 'a sign of and testimony to our faith' instead, thereby making in entirely understandable, and relating to us. (Now I'm not suggesting the Reformers were necessarily right, but I don't think that the move away from their vocabulary/theology was because of thought-through theological differences in this instance, but because of the spirit of the age - corrections welcome as always.) It's our natural attitude in evangelism: we will win them by explaining that it's altogether natural, straightforward and nothing mysterious at all. When I write it down like that, I think, "But no - it's not as if I deny the miraculous: I testify to the resurrection!" But I still always have the temptation and tendency to de-mysterialise. Do I mention the trinity when I speak of God? Do I shy away from speaking of the Holy Spirit? Or from mentioning the devil (or do I attribute everything to natural causes)? Am I happy to speak of just a part of the gospel at one time, leaving some mystery for future updates, or must I launch the complete system (according to me, copyright 2005)? Have I bought into the scientific ideal that we can naturally explain everything?
I'm not for mystery for the sake of mystery. I have heard some well-intentioned evangelism that was like launching an army of spiritual-sounding words each representing a concept so mysterious that the non-Christian was left clueless. That is clearly not the way Paul declared the mystery of the gospel, for example.
However, speaking not so particularly of evangelism but of church life in general, it could be seen in miniature in how we read NT use of 'mystery'. We seem to tend to react one of two ways:
1) we think "mystery -> we can't understand this; we must just experience it". This is a common tendency, but not what I'm addressing here - perhaps because, as a scientific mind, this doesn't tempt me much. See Paul's "I want you to understand this mystery" (Rom11.25), "according to the revelation of the mystery that was kept secret for long ages but has now been disclosed and through the prophetic writings has been made known to all nations" (R16.25-26), "I tell you a mystery" (1Cor15.51), "in all wisdom and insight making known to us the mystery of his will" (Eph1.8-9), and so on - various uses of 'mystery', but always refering to one revealed so that we know it and grow in understanding of it.
2) we think "mystery now revealed = no longer a mystery". That's not how it's used in the NT. It is still a mystery, but now one that God has revealed to us. It hasn't been made straightforward, still less has it been made natural.
We must stop trying to make the church something natural - a natural church isn't a church at all, but an ugly parody. The NT is persistant in referring to Christ and the church as a mystery revealed. It's all about Christ, who, revealed, is still a mystery: gloriously and savingly so.