We thankfully receive all of these, and other parallel, passages as clear and distinct proclamations of the gospel of divine grace and divine assurances of God's love for us and our eternal safety and security as we embrace Christ by faith and remain in that faith until death. They are wonderful assurances that remind us that God, who called us to faith and created saving faith in us through the gospel, will never leave or forsake us, but delights in preserving us as his dear children. Even if we show ourselves unfaithful, he remains faithful. In short, they are gospel assurances that comfort and encourage us in our pilgrimage toward glory.
You then ask what passages from the same sacred Scriptures tell us that it is possible for believers to fall from faith. In Luke 8 Jesus speaks of those who fall in times of testing and others whose faith is choked by life’s worries, riches, and pleasures. 1 Timothy 1 mentions those who shipwrecked their faith by not "holding on to faith and a good conscience." Other passages would include the following: Romans 11:20, 1 Corinthians 10:12, Galatians 5:2-4, and 2 Peter 3:17. These are generally proclamations of law and warnings for God's dear children not to abuse divine grace or trust in their own strength or merit, or to fail to value and make use of the gospel in word and sacrament.
We cannot contribute one speck to our salvation, but by our own arrogance or carelessness we can throw it away. Therefore, Scripture urges us repeatedly to fight the good fight of faith (Ephesians 6 and 2 Timothy 4 for example). My sins threaten and weaken my faith, but the Spirit through the gospel in word and sacraments strengthens and preserves my faith. That’s why Lutherans typically speak of God’s preservation of faith and not the perseverance of the saints. The key is not our perseverance but the Spirit’s preservation.
The Calvinist error of the perseverance of the saints theology is drawn not only from a failure to distinguish between law and gospel and to allow both divine messages to stand side by side as revealed, but especially from logical deductions stemming from their main error of imagining that God has from eternity elected some to be saved and others to be damned.