For through him we both have access in one Spirit to the Father.

According to Martyn Lloyd-Jones, it’s summarized by that very verse, Ephesians 2:18:

Indeed to me the statement before us is not only stupendous, it is staggering. ‘Through him, we both have access – access! – by one Spirit, unto the Father.” Our chief trouble, and the whole trouble with the Church, is that we do not realise the meaning of a statement like this. Were we to do so the Christian Church would be revolutionised. Were we to do so we should be lost in ‘wonder, love and praise’. We should realise that the most marvellous, wonderful thing that can ever happen to anybody in this world is simply his becoming a Christian.

If only every Church member, every Christian in the Church, realised the truth of this statement, the Church would be so different that we should scarcely recognise her. But oh, how different is the Church from what we find here! How many think of Christianity and of the Christian Church simply as a place which they attend now and again, and that perhaps in a perfunctory manner, hesitating, and doubtful whether they will or not, and as a matter of duty; or as a place in which they may exercise certain gifts that they have, and be busy – a kind of club, an institution, a human society. What a contrast to what we have here!

This is Christianity, this is what makes one a Christian. The Christian Church really consists of people who realise that this is the whole object and purpose of everything – access by one Spirit unto the Father.

– God’s Way of Reconciliation (Baker, 1972), pp. 245-46.