I believe that a Christian can lose their salvation. However, I hit a metaphorical stone wall when I read Romans 11:29. “For the gifts and the calling of God are irrevocable.” Salvation is referred to as a gift in the New Testament, so where do I go from here?
From the tone of your question, I can see that you believe that “All scripture is inspired by God” (2 Tim 3:16). When we are faced with what could at least be thought of as a contradiction, we should ask in what way they can agree. My experience is that when we do this:
1. We always find out that there is no real contradiction and that the two are easily found to be concordant.
2. We gain a newer and deeper understanding of God.
Let me apply this to Romans 11:29. Here Paul is talking about the Jews and the advantages the Jews had as his chosen people. If you go earlier, Paul explains that the Jews, who had a favored position before God because of his covenant, had experienced a hardening of heart in part. Many had become Christians but others had not. Those who had not accepted Christ as the Messiah had, on the whole, actually turned vehemently against him. This hardening had caused Paul to turn to the Gentiles, allowing thousands of them to come into the Kingdom of God. This was a good thing. However, Paul explains that, at least on some level, the Jews still have a special place in God’s heart. He still has hope that the Jews will be saved, in which case God will happily receive them into the Christian family. Either way, the Jews were the recipient of the covenant of Moses. They had more opportunity to know Christ than any others because of their background. Paul discusses this situation in Romans 11:11-12. Also, you can read Romans 11:1-6 in which Paul explains that God did not reject the Jews. “Yet I have reserved a remnant” (v. 4). They had many privileges. They did not lose those privileges when Christ came, but simply needed to accept Christ as savior. However, like us, if the Jews refuse Christ, they will be lost.
In summary, the gifts of God to the Jews (the prophets, the example of Moses, knowledge of God, the Old Testament) have not been lost by the Jews. They still have all these things. Neither has their call to saving faith been lost. Both their gifts and calling are still in force if they simply will do what the Old Testament was created to prepare them to do, which is to accept Jesus of Nazareth as their Messiah and savior.
So you see that there is no contradiction between this passage and the fact that Christians can fall away if they refuse to accept the gifts and calling which God still holds out to them.