By the experience of active love. Strive to love your neighbour 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 neighbour, then you will believe without doubt, and no doubt can possibly enter your soul. This has been tried. This is certain.
As love is full of unbefitting strains,
All wanton as a child, skipping and vain,
Form'd by the eye and therefore, like the eye,
Full of strange shapes, of habits and of forms,
Varying in subjects as the eye doth roll
To every varied object in his glance
The biggest embrace of love you'll ever make is to embrace yourself completely. Then you'll realize you've just embraced the whole universe, and everything and everybody in it.
One must learn to love, and go through a good deal of suffering to get to it, like any knight of the grail, and the journey is always towards the other soul, not away from it. . . . To love you have to learn to understand the other, more than she understands herself, and to submit to her understanding of you. It is damnably difficult and painful, but it is the only thing which endures.