And this is the testimony, that God gave us eternal life and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life. 1 John 5:11-12
It doesn't say, "believe now and then when your mortal body dies you'll have eternal life." It says you have it when you believe.
Paul says in Colossians 3:3, "For you have died, and your life is hidden with Christ in God." Notice that died is past tense, meaning my sinful nature has died and that is hidden is present tense. I am in Christ right now. It's Ephesians 2:6 that really blows my mind,
..even when we were dead in our trespasses, made us alive together with Christ--by grace you have been saved--and raised us up with him and seated us with him in the heavenly places in Christ Jesus.
Notice again that raised is past tense. Because we were forgiven in Christ's work on the cross, we were raised up and were seated with Christ. We're already there. Our kingdom is already in Heaven. Notice the writer of Hebrews 12:22-23 uses past tense again
But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to innumerable angels in festal gathering, and to the assembly of the firstborn who are enrolled in heaven and to God, the judge of all and to the spirits of the righteous made perfect.
God has already brought us into eternal life. I need to start living like that!