We can only know Christ through spending time with Him - listening to Him and meditating on His words Joh 5:39 Joh 8:31.
The Power of His resurrection eliminates fear Heb 2:15. In this fearlessness we are able to testify of Him to every man and so come to experience rejection, hatred and persecution Mat 24:9.
Such knowledge, power and sacrifice makes us conformable unto His death. We are living sacrifices Rom 12:1.
The world that Paul lived in needed Him as such. The world that we live in is no different.
When we deny ourselves and share the Gospel of Christ Jesus we enter into a level of fellowship with Him that those who do not submit never experience.