When we speak of offering spiritual sacrifices, we are not suggesting that Christ’s sacrifice was incomplete.
As declared in Hebrews 10 and Hebrews 9, Jesus Christ offered Himself once and for all fully and finally putting away sin. Nothing can be added to the finished work of the cross.
When 1 Peter 2 calls us a holy priesthood offering spiritual sacrifices, it is not referring to atonement it is speaking of worship.
As further revealed in Hebrews 13 and Romans 12, our spiritual sacrifices include:
Our praise to God
Our obedience to His Word
Our surrendered lives as living offerings
Our willingness to give and share what He has entrusted to us