In

John 14:20 ( NASB)

I am in you.

The greatest assurance a saint can have, and that which a heathen craves, is the abiding, indwelling presence of the Lord Jesus Christ in us. The peace, power, serenity, and absolute confidence that comes with the presence of God in us in the form of His Holy Spirit is the assurance and deposit guaranteeing our salvation. The lost and unsaved crave this assurance and confidence by going to holistic healers, psychiatrists, counselors, and such, but can't find it. They resort to drugs, alcohol, sex, and other vices to seek solace from their misery but can't find it. It is inconceivable that God can dwell in a mortal. But that is exactly what happens when a sinner repents and receives Christ. The fullness of God and His power live in us in His Holy Spirit.