3 What is meant by the words, “Because I live, you shall live also”? Why did He speak in the present tense of His own living, and in the future of theirs, but just by way of promise that the life also of the resurrection-body, as it preceded in His own case, would certainly follow in theirs? And as His own resurrection was in the immediate future, He put the word in the present tense to signify its speedy approach: but of theirs, as delayed till the end of the world, He said not, you live; but, “you shall live.”
With elegance and brevity, therefore, by means of two words, one of them in the present tense and the other in the future, He gave the promise of two resurrections, to wit, His own in the immediate future, and ours as yet to come in the end of the world. “Because I live,” He says, “you shall live also:” because He lives, therefore shall we live also. For as by man is death, by man also is the resurrection of the dead. For as in Adam all die, even so in Christ shall all be made alive. As it is only through the former that every one is liable to death, it is only through Christ that any one can attain unto life.
Because we did not live, we are dead; because He lived, we shall live also. We were dead to Him, when we lived to ourselves; but, because He died in our behalf, He lives both for Himself and for us. For, because He lives, we shall live also. For while we were able of ourselves to attain unto death, it is not of ourselves also that life can come into our possession.
Source: Tractates on the Gospel of John (New Advent)