Can I ever be sure that I'm saved?

Sometimes people who have been Christians for twenty or thirty years admit to me that they still don't know for sure if they're really saved.  The doubt may arise because they are so conscious of the sinful tendencies that still seem so entrenched in their lives.  Or, putting 'works' to one side, they're not sure if their faith is truly a saving faith.

The issue of ASSURANCE has bothered Christians ever since the earliest church.  Modern evangelicalism tends to downplay the issue by telling new converts that they are truly saved, just on the basis of a single prayer prompted by a counsellor.  Some church systems have virtually guaranteed salvation to their followers on the basis of attendance at services and performance of certain sacraments. 
But many others, including those in the Puritan and mystical traditions, have kept assurance separate from saving faith. 

The apostle John obviously wrote his first epistle to bring assurance to believers.  But the assurance comes across with a number of 'ifs'!  If we keep His commandments (2:3), if what we heard from the beginning abides in us (2:24), if we practice righteousness (3:10), if we love the brethren (3:14), if our heart doesn't condemn us (3:21). 

If we're worried about our walk with God, John's assurance may well make us even more unsettled!
There are also verses where Jesus Himself seems to make salvation conditional on enduring (Matt 24:13) or obedience (Matt 7:21).  The promises to believers are clear enough (John 3:16, 6:37, etc) but the assumption is that those who believe will go on to follow Jesus and do good stuff in His name.

For me, assurance of salvation is primarily about faith in the Gospel promise, God's Word.  But it's also about the knowledge that God's mercy and forgiveness are bigger than all my sins and failures.  When I see changes in me that can only be the work of God's Spirit, I am convincced that God will bring me through to full salvation.  After all, why would He bring me this far, only to abandon me in the end?

Finally, some professing Christians have no assurance of salvation because they are, in fact, not saved.  Perhaps they are trusting in themselves, or someone other than Christ.  Perhaps they have not truly repented or turned away from their ungodly lifestyle.

Let's keep things in their correct order.  There is no assurance without true repentance and faith!