Think of the consistency of the Christian faith. Take the five points of Calvinism, so called. Nobody even designed them as a method of teaching, and yet because they are based on Scripture, they fit so amazingly together. In fact it was the opponents of the Reformation who came up with the five points of Calvinism: actually they were expressing the opposite of each point. Those who were in favour of the doctrines of the Reformation simply turned them round the other way. As a result you get the so-called five points as a summary of key salvation doctrines. We speak of them are as the TULIP: T – Total depravity of mankind; U – Unconditional election to life; L – Limited election or particular redemption; I – Irresistible grace; P – Perseverance of the saints. Total depravity means salvation is all about grace. It doesn't teach that man is 100% depraved, but it teaches that in every department of his being there is corruption. There is some capability in him for good, but it is ruined by the bad. In every good endeavour there is a bad motive, and therefore not only can he not save himself, but he does not desire God and he will never seek after God. And how glad we are that God’s election to life in eternity past is unconditional, because if it was conditional there would be no hope for any of us. When Christ came, he did not die for everybody, so that everybody is potentially saved. It is not left up to us to determine whether we benefit from the death of Christ. No! This was a literal real atonement; Christ actually took the eternal punishment of sin for each one who is saved. He made everything secure. When God calls us to respond to the gospel, he does not leave it up to the fallible human will whether or not to respond. He as it were takes us by the hand, and we are made willing to come to him, so that it is our genuine response, but a response superintended by God. I would never submit to Christ if the Holy Spirit did not work irresistibly within me. Those who are made believers are kept by the Father and the Son and the Spirit. Even as a believer, there is still sufficient foolishness in me to fall from grace, but by the grace of God the Spirit works to ensure that I persevere in the faith all the way to the end. That is the order, the symmetry the connected nature of truth.
Some try to mix grace with other things. In 1977 the Bible believing party in the Church of England had a convention, which was held during the vacation at Nottingham University, and they gathered together under the chairmanship of the late minister of All Souls Langham Place, Dr John Stott. Among their deliberations they came to this conclusion: We are pretty well the same as the Church of Rome and we believe that all those people in the Church of Rome, even though they depend on Mary for salvation or the mass or the priesthood are to recognised as fellow-Christians. Even though they do not trust exclusively in Christ and his suffering and death on Calvary and repent and believe the gospel, but they also rely on good works and images and the intercession of dead saints, their way of salvation is just as good as ours, and it is just as valid. The conference made a declaration a statement in which they virtually said good works are sufficient as a means of getting to God, though we prefer grace. But a mixture of works and grace: it is all the same; everybody is all right in the end. You cannot do that! Opposites do not mix. You cannot mix grace and works.
How do I know if a doctrine is right, biblical? I know because it is in the Bible many times over. and it is never contradicted, and it is quite plain. There are some things, it is true, that we may find difficulty proving in the Scripture. We may feel sure that this is the right understanding, but if somebody challenged us or if someone had a slightly different view on that matter and they advance scriptural grounds for their view, we would not conflict with them. So there are some things, it is true, that are rather secondary, and while we think this is what they mean, we cannot be absolutely and utterly certain that everybody else is wrong. But all the important things in Scripture are repeated many times, and they are never contradicted, and they are solid and confirm each other. The great doctrines of the Bible are so plain, and you can organise them into a shape, and you can demonstrate that they fit, interlock and support each other. That is truth. Take one away, and there would be something plainly missing.