Like all other belief systems, reformed, evangelical, scriptural thinking is done within a system of basic tenets. There’s a set of interlinked biblical doctrines and attitudes which underlie it all and these are listed below.
About once a year I go back to this list as a really helpful way of identifying drift from reformed thinking to liberal theology. This exercise is taken from a lecture given by David Field at Oak Hill in 2005.
When liberal, non-reformed theology developed out of reformed theology during the enlightenment, a number of basic tenets were gradually eroded. Once eroded, these behave like a stack of dominoes, when one falls the rest follow. The key to avoiding theological drift is to identify our own Achilles heal. Mine is probably catholicity, wanting to be friends with everyone.
To do the exercise, read the following definitions of the tenets, find your emotional point of entry (the first domino to fall) and then work out which domino falls next. It helps to print off the diagram and draw lines from circle to circle.

Reformed theology to liberalism
Empiricism and inductivism: the belief that science trumps revelation. In this scheme, no human behaviour is inherently sinful, God is not offended by sin. Rather, all behaviour may be explained and changed using a pseudo-scientific method and counselling. And, of course, miracles are not miracles, they can be explained away. The incarnation of God and his resurrection from the dead are myths to the empiricist or inductivist.
Anti-voluntarism: Voluntarism states that God is good and so God’s will dictates what I do. If I think I know better than God, that’s anti-voluntarism.
Anti-determinism: if you don’t think God knows the beginning from the end and that all things work according to the sovereign will of God, that’s anti-determinism.
Anti-enthusiasm: Jeremy Paxman wrote “the English like their religion in moderation.” Instead of intensity, certainty and confidence you display a cool, arm’s length detachment to Christian truth.
(some) People are good (anti-total inability, anti-total depravity): When people only see the image of God in others, and overlook indwelling sin, they conclude “Surely God won’t condemn the good people”. Instead of admitting that people are unable to save themselves from sin and judgement, liberals conclude that God will let the “good” people in.
Reason verses revelation: For a reformed theologian the command to love the Lord your God with all your mind means; think as hard as you can about God revealed in scripture. For a liberal it means think independently as if human reason alone can fathom all mysteries.
Toleration: The bible teaches that God is not tolerant, he is patient. Patience expects change, tolerance doesn’t. The reformed pastor is patient with people but the liberal is tolerant of bad behaviour and/or sloppy theology.
Natural religion: liberals believe that God reveals himself only in creation and all religions sense this. Religion is an attempt by finite humans to put God into a system of words, codes and morals, therefore, all religions are equally inadequate attempts to grasp the ungraspable.
Catholicity and friendliness: the primary goal is for everyone to be friends, regardless of creed.
Anti-dogmatism: this is the belief that doctrine is bad because doctrine divides people. Best not to belief anything to strongly for fear of upsetting someone.
Hebrews 3:7-12 Therefore, as the Holy Spirit says, “Today, if you hear his voice, do not harden your hearts as in the rebellion, on the day of testing in the wilderness, where your fathers put me to the test and saw my works for forty years. Therefore I was provoked with that generation, and said, ‘They always go astray in their heart; they have not known my ways.’ As I swore in my wrath, ‘They shall not enter my rest.’” Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God.