I do think that all four theologians have very valid points and arguments, but I slightly side more with Barth’s views the most. Here are some points I agree with or find very interesting:
Tillich: [Proofs of God’s existence] are failures as rational arguments, and theology is ill advised to try to use them today to convince people of God’s existence. (p. 357); There are at least some formal similarities among all world religions and quasireligions. They are all expressions of ultimate concern and quests for human salvation. (p. 359)
Ecumenist: I do not think that we can prove the existence of God by a purely rational thought sequence. On the other hand, I do not think we can limit knowledge of God to the biblical revelation… I think we can and must carry on a discussion about God with everyone who is willing to listen – humanists, atheists, Marxists, and most certainly people of other religions. (p. 357); Christian theology today cannot be done responsibly in splendid isolation form other religions. (p. 363);
Barth: All Christian theology must have its center in God’s self-revelation in Jesus Christ… But we must not elevate our existential questions to systematic importance such that the revelation of Christ is allowed to speak only to these questions… we will discover that revelation questions us, reformulates the questions that we may have thought were so important. If we center on Christ, all of our questions will be included and addressed. If we insist on starting with our own urgent and often self-serving questions, we will probably end up with our own predictable and probably self-serving answers. That’s not revelation. (p. 359); When children open their packages on Christmas, their attention is upon the content of their gifts and not upon their remarkable capacity to receive. (p. 361); All human religion stands under the judgment of God. We take our religion, our dogmas, our rituals, our institutions, our moralities with frightful seriousness. But invariably at work is our arbitrary attempt to storm heaven, our secret urge to justify and sanctify ourselves. (p. 362); Jesus Christ is the only true way to salvation, not Christianity or the Christian church. (p. 365)
As you can see, I’m not much of a fan of Rahner’s position, particularly with his view about “anonymous Christians”. I’m quite sure that “all human beings are embraced by God’s love in Jesus Christ even if they know it” (p. 366 – 367), however, they at the bare minimum need to know the actions and sacrifice of Jesus for their sins. Their capacity to see or experience (divine or transcendent) love doesn’t necessarily lead them to conclude that there is a divine source behind those feelings or sense experiences. I see the point that all humans might share a basic, “universal” understanding of morality (i.e. that aggravated murder is wrong, cheating is bad, etc.), yet you need the specific revelation that Christianity (i.e. Incarnation and Resurrection) offers in order to be saved. However, I do remain mum on the state of mentally challenged persons and babies. But I remain open to the fact of God’s freedom and that He can choose to save whomever He wants.
However, I do believe that natural theology is not to be abandoned or be given the bad wrap that all the theologians give. I believe that we can and do learn a lot about God through the natural sciences, philosophy, and even other religions, but that alone doesn’t provide a saving knowledge of God as presented in the Gospels.