“To live is Christ” is made possible by the indwelling Holy Spirit. He is the giver of Christ’s peace, love, and joy to all who are in Christ by faith.
“To live is Christ” is to have the Holy Spirit, the Spirit of Christ, ministering the death and resurrection of Christ to my life. To be filled with the Spirit is to be filled with the life of the Lord Jesus Christ as a seed. He IS at work in us, Christian. And he is not waiting passively for us to meet a series of conditions before he begins his work. In fact, the work is already finished. We are fully alive in Christ. Fully justified. Fully sanctified. Fully glorified. Now the finished work of Christ is the ongoing work of the Spirit in us. By the Spirit, his death IS our life. And our dying is now our living.