[apothegm]

If p is undecidable in that case, then p ==> q is false. The meaning of the statement p ==> q is that p is never true without q also being true.

[slack3r]

You have to first prove that p is not undecidable. This is not being done in many proof by contra-positive based proofs in mathematics. First prove that p is not undecidable is not a step in Analysis I by Terence Tao for instance.

[FrankWilhoit]

Which letter of "if" do you not understand?