John 17
And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent
It's not just saying "I believe" it's truly knowing God and Christ and living by God's will through faith. And we only know God once we are born of him and have the Holy Spirit, (which is eternal life) dwelling within our hearts and by his grace, by the power of the Holy Spirit, he will strengthen those who truly love him faithfully and live by his will to overcome.
What do you mean by heart?