18 All this is from God, who through Christ reconciled us to himself and gave us the ministry of reconciliation; 19that is, in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting to us the message of reconciliation. 20Therefore, we are ambassadors for Christ, God making his appeal through us. We implore you on behalf of Christ, be reconciled to God. 2 Corinthians 5:18-20, ESV
If we cannot reject God's grace, why does Paul implore people to be reconciled to God? God reconciled the world to Himself through Christ. He did all that is necessary. Now, be reconciled to God.
Paradox is characteristic of Lutheran theology. We cannot by our own reason or strength come to faith on our own. It is not that God did His part for our salvation, now we have to do our part. We definitely have nothing of which to boast about that we did our part for our salvation. Yet, we can still reject Christ and reject the salvation that He worked for us. In my experience, when situations are paradoxical, the situation is not as simple as it first seems.