The phrase “under the blood” is an old fashioned one… not one most of us hear much in our modern church services - let alone outside of church! Occasionally the old song “Oh the blood of Jesus” may be sung so if you’ve been in church it may not be an entirely foreign phrase, though to many I’m guessing, it just sounds plain weird - though it really is kinda simple.
God is holy and pure...
We are not.
But we are loved.
Even with all the grot in our lives—we are loved beyond measure by this holy and pure God.
When we come to God accepting that Jesus blood was spilled to satisfy spiritual laws that necessitated payment, the wrongs that made us unclean are cleaned and before Him we stand as white as snow. The price paid in full.
ISAIAH 1:18 "Come now, let us reason together," says the LORD. "Though your sins are like scarlet, they shall be as white as snow; though they are red as crimson, they shall be like wool.
1JOHN 1:7 But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.
So, while none can claim innocence, and few may claim abstinence (in whatever degree) - all, under the blood of Jesus, are washed clean, made holy, and stand pure before God the Father.