That Christ has indeed died for our salvation and that by trusting in him and living as he wants us to live, we can share his life.
For instance, belief in the Trinity, the hypostatic union, salvation from sin.
Believers are people who have faith in Christ, trust their lives to him, believe fundamental truths about him and try to live their lives according to his wishes.
Which doctrine did you have in mind, Nan?
And prior to that, can you give me one piece of scriptural testimony anywhere in God's word that says I need scriptural testimony for a doctrine?
I agree that Israel is a legitimate interpretation but the imagery is polyvalent. I'm sorry you weren't encouraged to read or study the Bible as a Catholic. That's not my experience at all.
Revelation 12:
[1] And a great portent appeared in heaven, a woman clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars;
[2] she was with child and she cried out in her pangs of birth, in anguish for delivery.
[3] And another portent appeared in heaven...
It's true that Israel is certainly a valid interpretation of the woman in this passage; however, like many symbols, it is a polyvalent one and likewise relates to Mary. In fact, this woman who gives birth to the Messiah is also identified as the ark of the covenant, which fits Mary quite well...