Thomas a Kempis on Union with Christ

“The doctrine of Christ exceeds all the doctrine of holy men; and he who has the Spirit will find therein ‘the hidden manna’ (Rev. 2:17). But many who hear the Gospel of Christ have little desire for it, because they ‘have not the Spirit of Christ’ (Romans 8:9). But whosoever will fully and with relish understand the words of Christ must endeavor to conform his life wholly to the life of Christ.”

Thomas a Kempis, Of the Imitation of Christ

“You, however, are not in the flesh, but in the Spirit, if in fact the spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness. Romans 8:9-10

For more about the book of Romans see:

Martin Luther on the Book of Romans

Being a Biblical Christian, part 2

You Are Not Swayed by Appearances