The phrase "instantly believe" is correct and usable in written English.
It can be used when describing a quick or immediate acceptance of a statement or idea without hesitation.
Example: "When she told me the news, I instantly believed her; it was hard to doubt her sincerity."
Alternatives: "immediately accept" or "quickly trust."
Do most of us not instantly believe we recognize something quintessentially "Italian" in the preening vanity of Silvio Berlusconi, for instance?
There are no flashbacks to the war, and we don't need them; so fierce is Fassbender, and such is the glare of his gaze above the bristling military mustache, that we instantly believe in Tom as a veteran a disciplined soul determined not to crack.
Be smart, don't instantly believe everything you see on the Internet.
People's tendency to "google it" and instantly believe what they read can be an individual's or PR department's worst nightmare.
People who have ever visited Italy will instantly believe that this is true, as most Italian cities are capable of defrosting even the coldest hearts.
Mrs. Hobley said she instantly believed in his innocence.
He "instantly believed" that one of the attackers was "trying to cut the victim's head off", the court was told.
One bystander told how he "instantly believed that he was trying to cut the victim's head off" when he saw a man using a meat cleaver to mutilate Rigby's motionless body.
Another horrified bystander said he "instantly believed that he was trying to cut the victim's head off" when he saw the man using a meat cleaver to mutilate Rigby's motionless body.
Rolling Stone, however, instantly believed Jackie's story about a group of men brazenly plotting a felony, never mind a horrendously evil act.
When they heard that Obama -- a politician about whom they knew little, given his recent introduction as a national figure -- shared the faith of the terrorists who attacked us on 9/11, they instantly believed it because it sounded negative, blocking out all contrary evidence.
