'Sure' in the Bible

Therefore, believers, be all the more diligent to make certain about His calling and choosing you [be sure that your behavior reflects and confirms your relationship with God]; for by doing these things [actively developing these virtues], you will never stumble [in your spiritual growth and will live a life that leads others away from sin];

then [in light of the fact that all this is true, be sure that] the Lord knows how to rescue the godly from trial, and how to keep the unrighteous under punishment until the day of judgment,