Full lyrics posted here (click).
-Joe-

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The Guardian has a fascinating article based on the song and its rhymes, and a more user-friendly version of the lyrics:

It Ain't Necessarily so
(Ira and George Gershwin and whats-his-name, 1935)

It ain't necessarily so,
It ain't necessarily so -
De t'ings dat yo' li'ble
To read in de Bible -
It ain't necessarily so.


Li'l David was small, but - oh my!
Li'l David was small, but - oh my!
He fought big Goliath
Who lay down an' dieth -
Li'l David was small, but - oh my!...

Oh Jonah, he lived in de whale,
Oh Jonah, he lived in de whale -
Fo' he made his home in
Dat fish's abdomen -
Oh Jonah, he lived in de whale. .

In ain't necessarily so,
It ain't necessarily so.
Dey tell all you chillun
De debble's a villun
But 'tain't necessarily so.

To get into hebben,
Don't snap fo' a seben -
Live clean! Don't have fault!
Oh, I takes dat gospel
Whenever it's pos'ple -
But wid a grain of salt!

Methus'lah live nine hundred years,
Methus'lah live nine hundred years -
But who calls dat livin'
When no gal'll give in
To no man what's nine hundred years?

I'm preachin' dis sermon to show
It ain't nessa, ain't nessa,
Ain't nessa, ain't nessa,
Ain't necessarily so!

My sources say George & Ira were the songwriters, but I thought the lyrics for the Porgy and Bess songs came from a South Carolina playwright, whose name I forget just now.