II Corinthians 12:9

"And He has said to me, "My grace is sufficient for you.  For My power is perfected in weakness."  Most gladly, therefore, I will rather boast about my weaknesses, so that the power of Christ may dwell in me."


In the verse prior to this, Paul speaks of the thorn in his flesh, a messenger from Satan, concerning which he entreated the Lord three times that it might be removed.  Although answered with a "No", Paul was heeded and heard.  For, the Lord spoke to him.
We could easily miss a needed word here.  Many in our world balk at the thought that God could, can, or does communicate with any one.  The logic runs: "since I haven't heard His voice, He must not speak to anyone."  In a recent conversation I had with just such an individual, the gist of her perspective was -- "if I heard a voice in my head I would question my sanity."  And while I'm not insisting that the Lord speaks audibly these days (although He can certainly do as He sovereignly pleases)... He does most definitely speak to His children, as promised.
In the familiar 10th chapter of John, where the Lord Jesus pictures His relationship to believers employing Shepherd-to-sheep imagery, He promises speaking to His own:  "I have other sheep, which are not of this fold; I must bring them also, and they will hear My voice..." (v. 16); and again, some time later as He encountered opposition from the Jewish leaders, He asserted, "...you do not believe because you are not of My sheep.  My sheep hear My voice, and I know them, and they follow Me," (vv. 26, 27).
Coupled with this is Paul's teaching related to signs of sonship through the Holy Spirit, in Romans 8:  "The Spirit Himself bears witness with our spirit that we are children of God," (v. 16).  Apparently, through the Bible and through prayer, the Lord's Spirit assures His elect that they are His own, and need to continuously listen for His voice (Isaiah 55:2, 3a; Luke 10:39).
Now, what the Lord said to Paul wasn't the response he desired, but the promise he received was precious, indeed.  First, His grace is promised.  Grace is the undeserved, unearned favor that God sovereignly shows toward those upon whom He set His heart from all eternity.  Hence, grace is an unconditional attitude of love by which He has committed Himself to the eternal salvation and security of His elect ones.  Grace also has an enabling element... His grace empowers, imparting the ability we as His children constantly need in living out the Christian life of sanctification.  We were saved by His unmerited favor and power, and we are kept by such grace, as well.  
Secondly, grace's sufficiency is pledged.  The Greek present tense translated "is sufficient" infers an ongoing process.  The gracious Lord assured the tormented Paul that His empowerment, His grace, was not a one-time offer.  Thus, his faithful Father warmed Paul's exhausted heart by promising ongoing, fathomless, unceasing grace.  And since a promise is only as good as the one who makes it, every promise made by the omnipotent, immutable Creator of the universe CANNOT fail!  Note:  worry is not disguised doubt...it's unbelief.  We sin by worry in the face of circumstances, but how can we do so in the face of God's promises?  To worry is to impugn His character, cast doubt upon His faithfulness, and tell a watching world that He isn't completely trustworthy.
May we ponder His constancy by reflecting on the ways He has shown Himself strong on our behalf, and then refuse the encumbrance of fear & the sin of worry that so easily entangles us.  
Thirdly, His paradoxical principle is put forth: power perfected in weakness.  Paul's natural tendency (as is ours) was to be strong, self-sufficient, capable.  Instead, he found himself abused, ridiculed, and feeling forced to defend his apostolic credentials.  The thorn was the Lord's tool for highlighting Paul's dependence upon Himself, in order that His enabling grace would be most evident in Paul's life.  When thinking of God's power being manifest through weakness, Samson comes to mind.  Paintings consistently portray him as a "hunk", rivaling any muscle builder we can name... yet, it actually seems more probable that his physique was fairly average.  Consider: if he had been a rippling mass of muscles killing thousands with only the jawbone of a donkey, who would get the credit..?  Yet when done by a man of average build, observers wouldn't say, "Wow, that Samson is amazing!", but rather, "Wow, Samson's GOD is amazing!"  
So our lives are to be likewise lived in such a way that attention is drawn to our God, rather than to us.  We'll then find Him faithful, He'll gain the credit & glory, and a watching world will sense the difference He makes.