Community Chapel taught, and many ex-Chapel members still believe, that Trinitarians can’t define the Trinity using only biblical terms. This statement has of course been answered at length multiple times in multiple ways by myself and others, but I will answer it again here on this site for the sake of those who may still be swayed by it.
The best answer is that this charge is just flat wrong on the surface. The Trinity (Father, Son, and Holy Spirit) is all over the Bible and is easily defined using only biblical terms: the Father is God, the Son is God, and the Holy Spirit is God, and yet there is only one God. This is why the apostles were told in Matthew 28:19, “Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit.” Which of these terms is not Biblical? Anti-Trinitarians have their proof-texts that they say show the Son is not God or the Holy Spirit is not God, but their intrepretation doesn’t hold up when all the passages that speak to their identity are examined together and all are accounted as equally true. When all Biblical passages are taken into account, it becomes clear that the Son and the Holy Spirit are regarded as sharing with the Father in the unique identity of the one true God, the only Savior of His people.
On the other hand, if they say that God cannot be three and one at the same time—that it’s a logical contradiction—then they are attempting to bring God down to the plane of human existence. They are making God and His Word subordinate to human experience and reasoning.
The end result in both cases is that they have to discard the plain meaning of some parts of Scripture in order to maintain their various kinds of “Oneness” doctrine. But the meaning of the parts we’re supposed to keep is different depending on who you’re talking to.
One therefore also has to smile at the irony of this charge. As I have observed elsewhere on this site (“Unitarian Babel”), those who say the Bible doesn’t show us the Trinity cannot even agree among themselves on who the Bible shows Jesus to be. They all acknowledge him as the Son of God but they can't agree on what that means. The original "Jesus Only" folks (and today's United Pentecostal Church) say it means the one God in a different mode than the Father and the Holy Spirit. Chapel teaching, version 1.0 (i.e. what was taught at the Chapel before it split and then disintegreated), says it means a man who is also God. Chapel teaching, version 2.0 (i.e., what many ex-Chapel members have come to believe since the split), says it means a man who is NOT also God. Jehovah's Witnesses say it means a divine pre-existent being not equal to God the Father. Mormons say it means a God who started off as a man on the planet Kolob (look it up!) but whose obedience was rewarded with elevation to Godhood, the same as the Father and the Holy Spirit were also separate men who were elevated to Godhood. What are we to make of all this disagreement? I think I'm going to be pretty comfortable with the "Trinitarian version" of what the Bible says about the Son of God until all the opponents of the Trinity can agree on a version that has to be true instead.