Context
Paul knew that many members of Corinth's church weren't as spiritually mature as they should have been (1Corinthians 1:11 - 13, 3:1 - 5). Among his many concerns was the spread of false teachings and teachers among the brethren (11:1 - 2, 18 - 19, 12:3, 14:37 - 38). His frustration at their carnal behavior (4:21) and their gullibility to believe any smooth talking false preacher who visits them (2Corinthians 11:1 - 4) led him to using "Anathema Maranatha" at the end of his letter.
If any man love not the Lord Jesus Christ, let him be Anathema Maranatha (1Corinthians 16:22, KJV).
The curse Paul utilizes in 1Corinthians, written in 56 A.D., is the strongest correction he records toward the end of any of his fourteen Biblical books. He would later, in late spring of 58, utter a curse directed toward the High Priest that he immediately regretted (Acts 23:1 - 5).
Punctuation Clarification
It is easy to conclude, based on the King James punctuation of 1Corinthians 16:22, that Paul's curse entails the entire expression "Anathema Maranatha." This, however, is not true. What is missing in the KJV is a period between the two distinct declarations in order to separate them.
This error in the KJV makes it seem as if maranatha amplifies the curse of anathema. Most modern translations, however, correctly separate what the two words mean. That said, the incorrect connection and use of these two words has existed in some churches, such as the Catholics, as late as the early 20th century.
"In the Western Church, Maranatha has become a very solemn formula as anathema, by which the criminal is excommunicated, abandoned to the judgment of God, and rejected from the bosom of the Church until the coming of the Lord.
"An example of such an anathema is found in these words of Pope Silverius (536 - 38): "If anyone henceforth deceives a bishop in such a manner, let him be anathema Maranatha before God and his holy angels" (1913 Catholic Encyclopedia).
One accurate translation of 1Corinthians 16:22, which records Paul's maranatha curse but also his looking forward to Jesus' return, is the following.
If anyone does not love the Lord Jesus Christ, let him be accursed (anathema). Our Lord shall come (maranatha, 1Corinthians 16:22, HBFV)!