Once you have believed in Jesus, you have eternal life that can never be taken away. Jesus said, "My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand" (John 10:27-28). In Romans, Paul assures us that "neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord" (Romans 8:38-39). 

There is simply nothing you could do to lose the free gift of eternal life. The death of Jesus is sufficient to forgive all of our sins, even the ones we commit after becoming Christians (Hebrews 10:10-14). When you became a Christian, the Holy Spirit changed you in a permanent way (2 Corinthians 5:17). He sealed you, meaning that God promises to give you eternal life based on the work of Jesus alone (Ephesians 1:13-14). Furthermore, you were regenerated, which means that your heart and spirit were cleansed of sin and remade (Titus 3:4-5). Because of those irrevocable changes, there is no way you can lose your salvation. 

God is perfectly trustworthy. He promises save everybody who believes in Jesus, and He will follow through on that promise. What a blessed assurance, indeed!