Christians always agree on which doctrinal issues are essential and which issues are not.

No, they do not always agree.

The main doctrines that most Christians agree on are
(a) The deity of Jesus Christ, His virgin birth, and His sinlessness;
(b) The substitutionary atonement of Jesus Christ when e died on the cross;
(c) Salvation by grace apart from works;
(d) The presbytery or the structure of church government.

Doctrines that many Christians disagree on are
(a) The priesthood of believers (Christians);
(b) Speaking in tongues;
(c) Eschatology (doctrines about end times);
(d) God's promised future prosperity for Israel;
(e) The rapture, the order of resurrections, and the millennial reign of Christ on earth.