This post continues my series of short answers to common (and not so common) questions about Catholicism. For the previous parts in the series, see the "Catholic Q-A Series" blog label.
Is it true that blasphemy is an unforgiveable sin?
No. The only unforgivable sin is the rejection of the Holy Spirit, or perhaps despair. That’s because these sins compel a person to avoid seeking the forgiveness of God. But, if you actually confess your sin to God (in prayer or in the Sacrament of Reconciliation) with a contrite heart and a firm intention to amend your life, He will forgive it no matter what it is.
Is it sinful to have evil thoughts, even if you don’t carry them out?
It's not sinful to be tempted to do something bad or to have the sudden impulse to do something bad. We are tempted all the time to do things that we shouldn’t do. It is also common for a thought or a feeling to rush into your head that you know is uncharitable. We have to banish such thoughts and carry on. The sin is in carrying it out, like you said. It is also sinful to dwell on the sinful act or to relish in it in your mind, or to foster evil thoughts so that they grow and take root in your soul
In the Old Testament, especially Leviticus and Deuteronomy, it says that it is forbidden to eat certain foods. Do these laws apply to Christians?
Jesus said, "Do you not see that whatever goes into the mouth passes into the stomach, and so passes on? But what comes out of the mouth proceeds from the heart, and this defiles a man" (Mt 15:17-18). The parallel passage in Mark adds this explanation: "Thus he declared all foods clean" (Mk 7:18-19). In Acts, we read of a vision that Peter received where an angel showed him that just as food previously considered unclean is now considered clean, so should the Gentiles, who were previously considered unclean, now be considered as worthy of God (cf. Acts 10:9-16, 28). Finally, Paul wrote to the Romans, "Do not, for the sake of food, destroy the work of God. Everything is indeed clean, but it is wrong for any one to make others fall by what he eats" (Rom 14:20).
It appears from all this that the laws against clean and unclean animals no longer apply to Christians. The only time we would refrain from eating or drinking something would be if it caused scandal to our neighbor.
Does the Bible forbid marriages between Christians and non-Christians?
No. Instead it actually presupposes that mixed marriages will take place and encourages such couples to stay married because the believer may end up saving the non-believer by his or her own witness to the faith (cf. 1 Cor 7:12-16). Of course, the ideal situation is one where the spouses are of the same faith.