The form estō (ἔστω) is the third person singular present imperative of eimi — a command form meaning "let it be, let him be, let her be." It issues a directive about the state or character something should have. Rare compared to other forms of eimi, its appearances in the NT carry sharp ethical and theological force.
In James 1:19, "let every person be quick to hear, slow to speak, slow to anger" — the triple imperative sets out the disposition of the mature disciple. In 1 Corinthians 16:22, Paul writes "let him be anathema" — using estō to pronounce solemn exclusion. Matthew 5:37 uses the imperative form for ethical simplicity: "Let your yes be yes, and your no be no." The imperative of eimi commands a state of being — not an action, but an orientation, a character. You cannot manufacture it by trying harder; you must be transformed into it.