Jesus said, “Truly I say to you, there is no one who has left house or brothers or sisters or mother or father or children or farms, for My sake and for the gospel’s sake,30but that he will receive a hundred times as much now in the present age, houses and brothers and sisters and mothers and children and farms, along with persecutions; and in the age to come, eternal life." (Mark 10:29-30)
The fact is that even if you put aside the question of eternal life - which is supernatural and which is known to us only by Divine Revelation - even if you focus only on the meaning of these words in this life, they are profoundly and utterly true.
There is no greater guide to happiness in this life than the Way of Jesus and the lives of his saints.
There is no greater pragmatic guide to living, no greater preparation for reality, than the Gospel, than the way of the cross.
For the Gospel - and all of Scripture - and all of Church teaching - reveals one primary truth about how to live: from renunciation and sacrifice comes both compensation and persecution ... and eternal life - a kind of deep mysterious joy - begun in us even now.
That is simply the primary psychological truth about life.
And, really, you can't exclude the supernatural element. For the Kingdom of God is among us and within us; sanctifying grace and the Holy Spirit dwell like seeds or germs or leaven in our breasts, and can be cultivated for the fulfillment toward which they point.
Truths like these are found everywhere in Scripture. They operate on many levels and open themselves up to us in surprising ways at unexpected times.
And if it's only a guide book, it's the most remarkably accurate guide book ever written.