Sine qua non means "that without which there is nothing." Among my many blessings, only one is the sine qua non.
There are a thousand things for which I can and should thank God: family, friends, health, a degree of liberty, good food, housing, our president, the brave men and women guarding our liberty, a great pastor, a good church, really terrific bloggers like Phil Johnson, La Shawn Barber, the Discoshaman, and scads of others. The list of blessings could go on endlessly.
It is good and fitting to be thankful for these, as they are gifts of God (1 Timothy 4:4-5), and ingratitude is a sure sign of unbelief (Romans 1:21).
But all these gifts have one thing in common: they are fragile. Any one of them can be lost in a moment; and many of them will be lost, if I live long enough.
There is only one gift of God that is infinitely better in quality than the best of this list, and which will be mine for all eternity. That is Jesus Christ.
In Him I am reconciled to God. My sins are dealt with once and for all time, my hatred for God is turned to love, my death to life, my estrangement to adoption, my defilement to righteousness. God's name is written on me, and my name is written on Christ's heart. He holds me in the love of God, He will never let go, and no created thing has the power to separate me from His love.
Heartaches will come, old(er) age may come, loved ones will be parted from me, my health may fail, our country may fall -- but Jesus Christ is the same yesterday, today, and forever. In Him alone all the unfilled hopes and promises of the Old Testament are saved from being pious nonsense, and are brought to full realization; in Him alone all the real miseries of the human condition are met, reversed, and replaced with God's own glory.
I am terribly thankful for all the blessings in my life. But above them all, I am thankful for Jesus. Without Him, it all would be vanity of vanities, vexation of spirit, and striving after the wind -- on the best day!
(For more, please see Why I Am (Still) a Christian, and How Can I Know God?)