Those things which must be believed are all the dogmas, those things solemnly declared as infallible in Church councils or in papal "ex cathedra" statements (very rare, BTW). A second tier of church teachings exist on the level of "must not oppose". The lowest level is theological opinion, which people do not bear obligation to believe, and on this level discussion is encouraged. I may be missing some level(s), but that's the gist.

It should be helpful to point out that belief is not what saves, but the grace of God, which the Church knows she does not control but cooperates with God as a conduit. I say this because an infant has no capacity to assent to a dogma, nor any young child for that matter, but if we are to believe that the promise is made to us and to our children (Acts 2:39), some way other than belief must exist to attain salvation.

Salvation goes beyond the immediate scope of the discussion, but it is a worthwhile detour.

All that to say, some things a person must believe if he is able to intellectually assent (capacity and knowledge of the subject), but the will and mercy of God is not limited to a person's ability to believe.

> Acts 2:39: "For the promise is for you, for your children, and for all who are far away, everyone whom the Lord our God calls to him"