Having written the book on being Christian, of course Paul was Christian by our understanding, one who trusts his redemption to Jesus. Curiously, though, he probably never considered his faith anything but true Judaism.
Paul would have opposed those who proclaim, "I'm a New Testament Christian, not Old Testament." To Paul, the old was fulfilled in the new, not repudiated. It was the "perfect law, the law of liberty," not something to be modified or set aside.
He knew that works of law add nothing to the morality of Jesus by which we are judged. Yet keeping the teaching of Moses has its own value, revering it honors God, and trying to keep it brings out our need for Christ.
Paul would have opposed those who teach that the church has replaced Israel. Didn't he instead teach that believers in Christ are "grafted on" to Israel, and "joint heirs to the promises of Abraham?"
Paul rejected the burden of legalism. If he were here today, I think he would understand individual cases of syncretism by which Christianity has taken on symbols and customs of the cultures to which it has spread, and by usurping raunchy, pagan bacchanalias rendered them dear children's festivals.
But I think it would have cut his heart if he could see how perfectly the church today has abandoned all vestiges of Jewish festivals, symbols, days, diet and distinctives. If a church is going to have symbols anyway, reminders of truth, is it too much that they adopt authentic ones, prescribed in Scripture, as celebrated by Jesus and the apostles?
Again, I am not urging legalism. Far from it. I'm suggesting that to enjoy our salvation, cherish our love for Jesus, thank God for His word, live in our freedom, remove a stumbling block to Jews and emulate the first-century church. Wouldn't it profit us to stand in solidarity with Israel, practicing the same religious traditions that enlivened Paul?