'He called his twelve disciples to him' Matthew 10:1
This is the true inner secret to Christianity; being called to Christ Himself.
At the very heart of our religion there must come a true love for the person of Christ and for God the Father. Without this we serve only in half measures, our faith becomes lukewarm, and we follow Him at a considerable distance.
The word goes on to to say 'many are invited, but few are chosen'. (Matthew 22:14) - there are few who are truly willing to follow as Peter and Andrew did. 'At once they left their nets and followed him' Matthew 4:20.
I think in these verses there is found the whole kernal of the gospel and the true secret to real spiritual life. Leaving all at once to follow Him alone.