What does "remain steadfast" mean in the Bible?

Such as King David, after his sin with Bathsheba wrote in Psalm 51:10, “Create in me a clean heart, O God, and renew a steadfast spirit within me.” David realized that his sin had broken fellowship with the LORD God causing him to waver. He cried out to the LORD to make him immovable once again. The Apostle Paul, who faced one persecution and imprisonment after another, sought to encourage us all by writing in 1 Corinthians 15:58, “Therefore, my beloved brethren, be steadfast, immovable, always abounding in the Lord, knowing that your labor is not in vain in the Lord.” Our steadfastness must be rooted in the grace of our Savior, Jesus Christ. The writer of Hebrews said, “...For it is good that the heart be established by grace” (Hebrews 13:9). When we abide in Christ and remain steadfast in His grace, we will be ready to deliver the true hope of Jesus to others at any moment. “But sanctify the Lord God in your hearts, and always be ready to give a defense to everyone who asks you a reason for the hope that is in you, with meekness and fear” (1 Peter 3:15). Christian, fix your eyes solely on Jesus. Be established, deeply rooted, in His Holy Word. And be ready to give the Word to those around you regardless of your own personal circumstances. Remain steadfast!