"How should I witness to someone who belongs to a denomination, who I suspect isn’t trusting the Savior?"

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 person’s 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 person’s denomination, such as the virgin birth, the cross, and so on. One point of agreement will almost certainly be the Ten Commandments. They are the key to bringing any religious person to a saving knowledge of the gospel.

After someone is converted to Jesus Christ, the Bible will come alive and he will be led into all truth by the indwelling Holy Spirit. God’s Word will then give him light, and he will forsake religious tradition as he is led by God. While there are strong biblical arguments that may convince unregenerate people that their church’s 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, 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 church’s 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).