Use of "it" before "sufficeth to say"

The other answers are great; this relatively common usage should also be mentioned:

... but suffice it to say ...

"Sufficeth" is the equivalent of the modern-day "suffices", with an "s". This can be seen by other words which end with "-eth", such as:

He goeth to Morgantious./He goes to Morgantious.

Now, we could say "Go to Morgantious", but we can't say "Goeth to Morgantious" because we can't say "Goes to Morgantious."

Thus, in the same way, we can't say "but sufficeth"; we would need a pronoun:

but it sufficeth to say...

In the same way, we would write:

but it suffices to say...

“Sufficeth” is just an old spelling of “suffices” commonly used in the King James translation of the Bible and other Renaissance religious texts. People often use it in a joking manner to give their writing a semi-Biblical air, especially in the phrase “it sufficeth to say.” But they sound clumsy rather than clever when they omit the “it” and begin the phrase thus “Sufficeth to say....”

“Sufficeth” is a verb; it requires a subject.

It means:

be adequate, sufficient, enough and capable