An apologist is someone who makes it their work to defend a position, usually one that is under constant barrage from critics. In this case, proving the existence of a Christian God. Now that we know the context, we can examine the poem itself within that context.
From all my lame defeats and oh! much more
From all the victories that I seemed to score;
From cleverness shot forth on Thy behalf
At which, while angels weep, the audience laugh;
From all my proofs of Thy divinity,
Thou, who wouldst give no sign, deliver me.
Taken in context, it begins to make more sense. Lewis is reflecting on his attempts at refuting critics of Christianity. Sometimes he fails, sometimes he succeeds. The last verse is a call to God to help out his cause, to provide solid proof of His existence, yet He does not.
Thoughts are but coins. Let me not trust, instead
Of Thee, their thin-worn image of Thy head.
From all my thoughts, even from my thoughts of Thee,
O thou fair Silence, fall, and set me free.
Lord of the narrow gate and the needle’s eye,
Take from me all my trumpery lest I die.
The second verse is a bit more esoteric. Taken as a whole, it is not so much a confession of defeat, although it makes use of defeatist language, but an admission that at the heart of Christianity is faith. Faith within religion is a complete trust regardless of the existence of material proof. "Let me not trust, instead Of Thee", "O thou fair Silence, fall, and set me free.". The fifth verse is a reference to Bible verses, and the last verse is a call to deliver them from their own mortal cleverness in defending their position.
This is my own interpretation and could likely be completely wrong, so take from it what you will.
It is interesting, but why is it in the Android community?