As Jesus was crucified he uttered those memorable words to shake the very foundation of all those powers which opposed him: "Father, forgive them, because they don't know what they are doing."  Or did he?

This is a quote of Jesus known and loved by many.  People who know little else of Scripture seem to know this quote of Jesus.  However, there is early and wide manuscript evidence suggesting these words do not belong to the Gospel of Luke [P⁷⁵, א¹, B D* W Θ 070 579 1241 pc sy^s sa].  This is not to say that it shouldn't be regarded as a genuine saying of Jesus since there is also wide and early support for its inclusion [א*^,2 (A) C D² L Ψ 0250 Ë^1,(13) 33 Ï lat sy^c,p,h] -- on which see Bruce Metzger's Textual Commentary on the Greek New Testament (2^nd Ed., UBS 1994, p.154). 

Arguably, these words may have been borrowed from the similar saying of Stephen as he was being stoned (Acts 7:60).  This would certainly explain a reason for the inclusion in Luke and not in the other Gospel accounts.  However, it still seems like something which might be included in Luke given the teaching of Jesus on forgiveness found in Luke 6:27-36. 

The question I would have about this statement is whether it should be allowed to play such a prominent and important role influencing our theology?  What should be made of the attribution of ignorance to those killing Jesus?  Would it not serve us better to make our case for the forgiveness of sinners from other passages instead of focusing upon one that is ambiguously original to the Gospel?