By the experience of active love. Strive to love your neighbor actively and indefatigably. In as far as you advance in love you will grow surer of the reality of God and of the immortality of your soul.
 Fyodor Dostoyevsky, The Brothers Karamazov (1880). copy citation

Context

“I stand [pg 056] and look about me and see that scarcely any one else cares; no one troubles his head about it, and I'm the only one who can't stand it. It's deadly—deadly! ” “ No doubt. But there's no proving it, though you can be convinced of it. ” “ How? ” By the experience of active love. Strive to love your neighbor actively and indefatigably. In as far as you advance in love you will grow surer of the reality of God and of the immortality of your soul. If you attain to perfect self-forgetfulness in the love of your neighbor, then you will believe without doubt, and no doubt can possibly enter your soul. This has been tried. This is certain. ” “ In active love?” source