Lincoln was steeped in Biblical knowledge. He peppered his speeches with quotes from the Bible. He made poetic allusions to biblical imagery too.
In his speech when he was named the Republican Illinois candidate for the US Senate, he thundered, “A house divided against itself cannot stand,” inspired by the phrase that appears in Matthew 12:25, Luke 11:17, and Mark 3:25.
In his 1st Inaugural Address, he concluded by appealing to ‘the mystic chords of memory…the better angels of our nature, telling Americans we are not enemies, but friends.
In his 2nd Inaugural Address, he concluded, “With malice towards none; with charity for all…, as God gives us to see the right…to bind up the nation’s wounds…”.
But he was a skeptic of God for most part of his life. He was not a fervent believer. But with his Gettysburg Address, he finally acknowledged the existence of God and his benevolence.
In his 3rd draft of the speech, in the conclusion, he inserted “under God” to make his speech read “That this nation, under God, shall have a new birth of freedom…”. [p 175 Mansfield].
There is some controversy on this addition of the words “under God.” In The Eloquent President by White, he stated that the words were an extemporaneous addition when he delivered the speech. [p 250 paperback]