Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:

Ye also, as lively stones, are built up a spiritual house, an holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ.

COMMENTARY

Purge out therefore the old leaven, that ye may be a new lump
To offer up spiritual sacrifices, acceptable to God by Jesus Christ
Continuing the point from before, this new verse also speaks of how we are meant to change our old nature to align with Christ. And this change necessarily requires some sort of sacrifice. Quite simply we cannot put on a new piece of our Savior, until we have first made space for it by getting rid of something else. Thus, sacrifice.
This is why animal sacrifice was never sufficient to save souls, for it did not require any actual change. Those that came to the temple to only give up an animal missed the whole point of it. That offering was always meant to be coupled with one of these “spiritual sacrifices” that Peter mentions.

Even Christ our passover is sacrificed for us
When we see the sacrament prepared on the altar, we are witnessing what God is willing to offer to save us: the life of His son. But if we eat and drink, say “thank you,” and leave, then we, too, are missing the entire point. Because God’s offering was always meant to be matched with another…the “spiritual sacrifice” that we bring in return. The ancient Israelites never came to the altar empty handed, and neither should we.
Of course what we offer will not be on the same order of magnitude as what God has. That’s alright, it doesn’t have to be equal. But we should be offering something. Perhaps we could be giving up an old sin? Perhaps we could be making a new commitment for good? Just so long as it is something which states “I see what you have given up for me, God, and I will honor it by giving up my own inferior part, and taking the better one that you offer to replace it.”