Yes, Jesus did completely atone for our sins. We can add nothing of merit to His accomplished work. 1 Timothy 2:5-6 NKJV, “For there is one God and one Mediator between God and men, the Man Christ Jesus, who gave Himself a ransom for all…”
However, Jesus said, “If anyone desires to come after Me, let him deny himself, and take up his cross daily, and follow Me.” Luke 9:23. Self-denial is sacrifice. The Apostle Paul also wrote, “I now rejoice in my sufferings for you, and fill up in my flesh what is lacking in the afflictions of Christ, for the sake of His body, which is the church,” Colossians 1:24 NKJV.
Why does the body of Christ need to suffer? God wants a divine family. God resurrected Jesus with the divine nature as a reward for sacrificing His own will. But God also wants the saints to share in the divine nature. Our obedience through sacrificing our human wills to do God’s will develops our characters to qualify us for the divine resurrection. Psalm 50:5 KJV – “Gather my saints together unto me; those that have made a covenant with me by sacrifice.”
Even Jesus needed to suffer (deny His own will) to prove His character as a new creature. The New Living Translation of Hebrews 5:8-9 explains this concept well. “Even though Jesus was God's Son, he learned obedience from the things he suffered. In this way, God qualified him as a perfect High Priest…”
The whole Christ, Head (Jesus) and (body, the saints), will be qualified to be sympathetic high priests and kings over mankind during the Millennium. Revelation 5:10 KJV, “And hast made us unto our God kings and priests: and we shall reign on the earth.”
For a more thorough study about the calling of the Christian, download the free PDF: "Why Are You a Christian?"