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.
The greatest gift a parent can give a child is unconditional love. As a child wanders and strays, finding his bearings, he needs a sense of absolute love from a parent. There's nothing wrong with tough love, as long as the love is unconditional.
Love others as you would love yourself, judge others as you would judge yourself, cherish others as you would cherish yourself. When you wish for others as you wish for yourself and when you protect others as you would protect yourself, that's when you can say it's true love.
If you are penitent, you love. And if you love you are of God. All things are atoned for, all things are saved by love. If I, a sinner even as you are, am tender with you and have pity on you, how much more will God have pity upon you. Love is such a priceless treasure that you can redeem the whole world by it, and cleanse not only your own sins but the sins of others.