The suspect in the Chabad of Poway synagogue allegedly wrote a seven-page letter saying Jewish people killed Jesus and deserved to die.
The 19-year-old is accused of killing one woman and injured several others in the attack on the shul, near San Diego, California, on the last day of Passover.
John Earnest - who has pleaded not guilty to murder and attempted murder - appears to have written the document in which he said his intention to do so would glorify God.
The revelation has led to religious leaders in the Orthodox Presbyterian Church movement, of which Earnest was a member, condemning the attack and document.
In a statement, leaders for the OPC church said: “Antisemitism and racist hatred which apparently motivated the shooter . . . have no place within our system of doctrine."
Speaking to The Washington Post, Rev. Mika Edmondson, who is a pastor in the Orthodox Presbyterian Church, said he read the words in the letter and was stunned.
“It certainly calls for a good amount of soul-searching,” Rev Edmondson said.
The Orthodox Presbyterian Church was founded to counter liberalism in mainline Presbyterianism.
Earnest is a member of Escondido Orthodox Presbyterian Church in the San Diego suburbs.
It is understood that his father was a elder in the church and regularly attended services.
Rev Edmondson said he believed the shooter “was radicalised into white nationalism from within the very midst of our church.”
When the shooter entered the building and opened fire he shot the rabbi, Yisroel Goldstein, in the hand.
Lori Gilbert Kaye, who was killed, has been hailed as a heroine for putting herself between the shooter and the rabbi, saving his life.
Members told reporters that they were shocked and horrified at his alleged actions.
The Washington Post said the Pastor vowed to see how he could support the grieving synagogue members.
Mr Earnest, who had five siblings, is believed to have said his beliefs did not come from his family.
His family published a statement saying their son’s beliefs horrified them and were not views they shared.
San Diego county sheriff William Gore said investigators were reviewing social media posts and an “open letter” published before the attack.
It is understood the document, which was posted to online forum 8chan, was littered with antisemitic and racist language.
It was said to be similar to the one published on the same website by the suspect in the shooting inside two mosques in Christchurch, New Zealand in March.
