In a video interview, Tim Keller made the issue of orthodoxy and heresy a matter of affiliation to the Apostles' Creed, and vaguely implies acceptance of the Nicene Creed as well in the allusion to acceptance of the Ecumenical Councils (which for at least the Reformed mean the 4 councils of Nicea, Constantinople -1st, Ephesus and Chalcedon). This sort of "mere Christianity" comprise the essentials of the faith, and therefore Keller believes that Christianity should be a broad enough tent to accept theistic evolutionists of all sorts.
The problem of course is that nobody actually practices that kind of "mere Christianity." Even Tim Keller does not. The whole idea of having a "Gospel Coalition" implies that they are making something i.e. the Gospel, a matter of orthodoxy. Where exactly is the Gospel explicated in any of the ecumenical councils? The Gospel was assumed by the church fathers and kept to the basics of belief in Christ, but beyond that there was not much development on what the Gospel actually is and what belief in Christ means.
The doctrine of Justification by Faith alone is not mentioned by any of the ecumenical councils. In point of fact, the Late Medieval Franciscan Pactum of Gabriel Biel (Facientibus quod in se est Deus non denegat gratium) is a denial of Justification by Faith alone. If Keller was truly aiming for "mere Christianity," he should say that the entire idea of the Gospel Coalition is a charade and embrace not only the rest of professing Evangelicalism (including the National Association of Evangelicals) but also the liberals (National Council of Churches), the Roman Catholics and the Eastern Orthodox. After all, all of them except perhaps a few radical liberals profess belief in the ecumenical councils of the Church.
So is Keller serious about his idea of "mere Christianity"? If he is serious, he should just stop the charade. Leave the Gospel Coalition, leave the PCA, and be an all out ecumenist! Be honest to your beliefs. Either fully embrace the idea of "mere Christianity," or man up and state clearly a position which one will live consistently by.
C.S. Lewis, for all his literary genius, is not an authority on the Christian faith. One can benefit from critically reading his works, but Lewis is not an authority on a lot of issues, the idea of "mere Christianity" being one of them.
To put it as simply and bluntly as possible, choose this day which one you will embrace: the Gospel (and all that comes with it), or "mere Christianity."