Though God be Christ’s Father, as second Person, yet they are all equal in power, dignity, and glory; 41but as Mediator, God is his Father in another respect. So it is said, John xiv. 28, ‘My Father is greater than I’—not as God, for so he was equal; ‘He thought it no robbery to be equal with God:’ Phil. ii. 6. But ‘greater than I;’ that is, consider him as man and mediator, in the state of his humiliation; for it is notable to consider upon what occasion Christ speaks these words: ‘If ye love me ye would rejoice because I said I go unto the Father; for my Father is greater than I;’ that is, You admire me and prize my company exceedingly, because you see the power which I put forth in the miracles which I do; ye would rejoice if you understood it aright; he is infinitely more glorious than I appear in this state of abasement and humiliation. Thus, with respect to Christ, God, the first Person, may be called the Father.
See http://www.ccel.org/ccel/manton/manton01.i_3.html#i_3-p21.2 for the context of this quote.
For some historical background on Manton, see http://www.ccel.org/ccel/manton