The Bible says so! If you believe that Jesus is the Lord, the Son of God, and that His death and resurrection is proof of His claims, you can learn how to be sure you're saved. The word of God leaves no doubt or question concerning this outcome. According to the Bible, those who profess faith in Christ receive a down payment on their eternal inheritance, which is the Holy Spirit taking up residence in our hearts.
Rom. 10:9 that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved;
Eph. 1:13 In Him, you also, after listening to the message of truth, the gospel of your salvation — having also believed, you were sealed in Him with the Holy Spirit of promise.