People can only know whether they are predestined from their having persevered to the end. God has included in the membership of the church some people who are not elect and do not persevere in order that the predestined should not be proud and seek out a false peace.
Herman Bavinck, Reformed Dogmatics, v. II, p. 351
The book of Hebrews is noted for having multiple warnings against falling away from the faith. While the above quote does not come from a discussion about the book of Hebrews, I believe it is a helpful guideline for those who might struggle with some of the difficult passages the book contains.
The Bible speaks very clearly about the eternal security of the believer. It warns just as clearly about the danger of falling away. Bavinck’s statement does a good job of reconciling those two strands of data.