That word ‘by’ might have misled us. What it means is that through the church, by observing God’s work towards the church, the angels learn more about the wonders of grace than would be possible in a world with no sin, no redemption, no forgiveness and salvation.
Think of John Bunyan, the author of The Pilgrim’s Progress. There he is in simple clothing, standing on an upturned box in a draughty barn. There are hundreds of people crammed in to hear him. There is no orchestra, no choir, no multi-decibel sound system; there is nothing to impress people at a human level. There is just a country preacher and there he preaches and the power of God comes down and people are saved and come to Christ, and it is a demonstration to the angels of the wisdom of God. The word of God pierces the heart, opens the mind, humbles the sinner, convinces him, brings him to repentance. Noise, and clamour, and human exhibition would conceal all that. So may the churches not adopt the paraphernalia and the gadgetry and the supposed ‘power and wonder’ of modern ways in any shape or form.