Martyn Lloyd-Jones delivered two sermons about cults while expositing Ephesians, one titled Cults and the other Counterfeits. These are easily two of the best sermons I have ever heard, and I strongly recommend listening to them (or reading them, since his sermons on Ephesians were compiled into an eight-volume set). The following is a quotation from the second of the two sermons, found in volume seven:
Any teaching that tells you that you can have any blessing apart from Christ is a denial of this selfsame truth, because every blessing that comes to us from God comes in and through the Lord Jesus Christ. Notice how Paul writes to the Colossians: ‘That their hearts might be comforted being knit together in love, and unto all riches of the full assurance of understanding, to the acknowledgment of the mystery of God, and of the Father, and of Christ; in whom are hid all the treasures of wisdom and knowledge’ (Colossians 2:3). He says again: ‘Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ; for in him dwelleth all the fullness of the Godhead bodily’ (Colossians 2:9). Every blessing that ever comes to man comes solely in and through the Lord Jesus Christ, and any blessing that is offered you apart from Him and His death and His blood is ultimately a denial of the faith.
This is why I speak with passion, as we all should. It is an insult to Him, it is an insult to His all-sufficiency. There is no need of the cults because everything they offer, and more, is given in Christ. They have no right to exist. They are an insult to Him, their very existence is an insult to Him. There is nothing He cannot give. He is ‘the Alpha and Omega, the beginning and the end’; He is ‘the first and the last’; He is ‘all in all’, and ‘we are complete in Him’. A man who tells me that Christ is not enough, that he needs some other formula in addition, is insulting Christ, is denying Him. Christ has everything, because He is everything. The Apostle tells us: ‘I know both how to be abased and how to abound; everywhere and in all things I am instructed both to be full and to be hungry, both to abound and to suffer need. I can do all things through Christ which strengtheneth me’ (Philippians 4:12, 13). Or again, ‘In nothing be anxious; but in everything by prayer and supplication with thanksgiving, let your requests be made known unto God. And peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus’ (Philippians 4:6, 7). If you are even considering the cults and wondering whether they can help you, you have already denied Christ. There is nothing that He cannot give you, He is everything.pp. 144-145