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The alter call might provide more assurance of an actual convert because 1.) they came to you and 2. You have follow up that you don't have on the street in particular.
So why street preach? I don't know, because that's how it was done once upon a time? I would say if done well it gives a call, a general call. I have seen a couple videos of that Todd Friel guy and he didn't seem terrible, he got a few good points across I thought.
I see nothing wrong with it. I do believe we are to share the Gospel. We were created to do so I think.
You said Calvinists can prove the prophecy doesn't pertain to us? How do we know?