I believe the difference is that when you call upon the Lord, you should be doing so with the full intention of accepting Him as your Savior. And when you accept Him as your Savior, you should be accepting that He died for your sins, you should be accepting all of His teachings, you should be accepting His Will in your life, you should be accepting the example He gave us as His followers. And part of accepting His teachings means believing in baptism, confession through a priest, recieving the Eucharist (Body, Blood, Soul, and Divinity of our Lord), turning away from from the influences of the world, etc.
When Jesus says that not everyone who calls "Lord, Lord" shall enter the kingdom of Heaven and He responds to them "I do not know you", I believe this is in reference to those who believe they can merely say the words and not truly accept ALL of Him. You can't merely accept one thing and omitt the rest. You can't say that you believe He is the Savior and seperate Him from His teachings and His example. Everything He said was important and said for a reason and we need to believe in all of His teachings.
This is not to say that all who aren't Catholic aren't saved. There are so many protestants who truly seek to know the truth, who truly seek Him and His Will in their lives. There is so much that the Lord takes into consideration and He knows their hearts. I'll leave the judging who is saved and who isn't up to Him. But I believe that is the difference between the scripture posted...
13 For whoever calls upon the name of the Lord shall be saved.
Matt 7:21
"Not every one who says to me, `Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of my Father who is in heaven.
In Christ,
Bridgette