If that's true there's no reason to witness, no reason to repent, no reason to get saved, no reason to do anything but what you wish it's all been decided for you.
Faith is central, faith is key. Without faith none of this happens. But faith itself is the gift of God (Ephesians 2:8), faith which comes not by human power, but by the power of God in the Gospel (Romans 10:17). So there is every reason to preach the Gospel, for it is through Word and Sacrament that God works His grace--giving us faith, justifying us, sanctifying us, and saving us.
Further, the elect can resist, the elect can fall away. This isn't single predestination Calvinism; in which case it would basically mean fatalism.
Calvinism amounts to determinism: God determined you to be saved, so you're going to be saved.
That's simply not what Lutherans mean by predestination.
Of course, as long as one tries to make everything logically consistent, the biblical teachings on these things are going to be very difficult to accept. Because we have to accept that it doesn't make sense logically.
God predestines us, having elected (chosen us) unconditionally in Christ before the world began. He chose us, we did not choose Him.
Yet, while we cannot say "yes" to God to get saved, since man is sinful and estranged from God; man can and consistently does say "no" to God.
Thus the irresistable grace of God is resisted. Does that make sense? No. But then again, the Incarnation doesn't make sense either. How can God, who can neither die nor suffer, both die and suffer? If we try to answer this with Nestorianism (only the humanity suffered and died) we fundamentally reject and deny the Incarnation itself. The Incarnation means the impossible: The Immortal God died, the Impassible God suffered.
God who cannot die, died.
God who cannot suffer, suffered.
The Christian will find that embracing the mystery and paradoxes of faith, rather than fighting against them, will be far more at peace than the one who insists on getting all the theology worked out right like some mathematical formula.
It should be sufficient that Christ says, "With man this is impossible, but with God all things are possible" Which Jesus says directly and specifically about our salvation: It is impossible for man to be saved, except God turns the impossible possible. That's the Gospel. The impossible salvation of helpless and hapless sinners like you and me, and the full redemption of this world from death by the renewal of all things which we hope for.
-CryptoLutheran