The most effective way to speak about the issues of eternity to a religious person is not to get sidetracked from the essentials of salvation. Upon hearing a persons background, we may feel an obligation to speak to issues such as infant baptism, transubstantiation, etc. However, it is wise rather to build on the points of agreement between the Bible and the persons denomination, such as the virgin birth, the cross, and so on.
While there are strong biblical arguments that may convince unregenerate people that their churchs traditions contradict Holy Scripture, there is a difficulty. Some religious people hold the teachings of their church to be on a par with, or of greater authority than, Holy Scripture. It is therefore often futile to try to convince them intellectually that their trust should be in the person of Jesus Christ, rather than in their own righteousness or in their church traditions.
For this reason we should aim at the conscience, rather than the intellect. Take them through the Law of God (the Commandments) to show that they are condemned despite their works (See “Am I good enough to get to heaven?” as an example of this approach), and strongly emphasize that we are saved by grace, and grace alone, rather than by trusting in our own righteousness or religious traditions.
If they are open to the gospel, and are interested in what God's Word says in reference to their churchs teachings, they will listen to Scripture. For example, in Matthew 8:14 we see that Peter (whom the Roman Catholic church maintains was the first pope) was married, as were many of the other apostles (see 1 Corinthians 9:5).