His Doctrine was Jesus Christ

'His matter was never trivial, and he never wandered into idle rhetoric. To expound the Scriptures before him as closely and as clearly as he could, then to bring the message to bear full on the conscience and will of the hearers, was his settled aim, first kept in view intelligently and with great pains. And what was his doctrine? In two words, it was Jesus Christ. Everything from Simeon's preaching radiated from Jesus Christ, and returned upon him. Not that he forced texts away from their surroundings, and forgot the literal in the mystical. But he was sure that Christ is the burden of the worlds of the Prophets and the Apostles, and he knew that He was everything for Charles Simeon.

Mere moral essays in the pulpit were for him impossible, though no man could well hold the standard of virtue and duty higher than he did. And so were merely critical discussions, though he always stimulated his hearers to think. For him,. Christ was the centre of all subjects for sinful men, and all hius heaerers were sinful men, for whom the Gospel was the one remedy. Christ was the Gospel; and personal faith in Him, a living person, was the Gospel secret. To Christ all men were called, for 'pardon, and holiness, and Heaven; and those who came at that call belong henceforth to Christ, His property, bound to live and die to their Lord.' Simeon himself thus described the three great aims of his preaching: 'To humble the sinner, to exalt the Saviour, to promote holiness.' 

Charles Simeon: Pastor of a Generation. Handley Moule, Pp50-51