“For by one offering He has perfected forever those who are being sanctified” (Hebrews 10:14 NKJV).

When we have received Christ’s “offering,” we are already “perfected forever.” We are already made whole and complete in Christ, so that we are fully accepted by the Father for eternity. Positionally, we are already at the right hand of the Father in Christ. We are already holy. We are saints.
Yet, we are still “being sanctified. We still experience the battle between the flesh and the Spirit, and sometimes give into the flesh. But all our sins are already forgiven. The Spirit empowers us to repent and walk in His power, growing in maturity. This is our experience. This tension between our position and our experience has been called the “already/not yet” of the Christian life. In Christ, we are already perfected, but not yet perfect.
But on that Day, our position and our experience will be the same, for we “shall be like Him” (1 John 3:2). Those that are in Christ, will become like Christ. This is God’s purpose for those who believe.