# For certain men have slipped in secretly among you

"For some men have come in among the believers without drawing attention to themselves"

# These men were marked out for condemnation

"These were men whom God chose to condemn" 

# who have changed the grace of our God into sensuality

"who teach that God's grace permits one to continue to live in sexual sin" 

# deny our only Master and Lord, Jesus Christ

Possible meanings are 1) they teach that he is not God or 2) these men do not obey Jesus Christ.