From Theology and Sanity, first published in 1946 and revised in 1978:
[The Apostles] knew Christ before they knew He was God. Had they known from the beginning, they might simply have feared Him, and fear would have made a bar to any progress in intimacy. But by the time they knew beyond the possibility of uncertainty that He was God, they had come to know that He was love. If they had known that Christ was God first, then they would have applied their idea of God to Christ; as it was, they were able to apply their knowledge of Christ to God. The principal fruit for them and for us of their three years of companionship with him was the unshakable certainty of His love for mankind; and it was St. John, the Apostle He loved best, who crystallized the whole experience for us in the phrase of his first Epistle, “God is Love” (4:8).
We may ask why the Jews did not know that already, for God had shown them His love often enough; and in the Old Testament His love is wonderfully stated. “The Lord is compassionate and merciful, long-suffering and plenteous in mercy” (Ps 102:8); that is strong enough, yet it is not the strongest thing of its sort. In Isaiah (49:15) there is a phrase which would seem to reac the very limit of divine tenderness: “Can a woman forget her infant, so as not to have pity on the son of her womb? And if she should forget, yet will I not forget thee.” The truth is that love arises and abides most easily and naturally where there is community of nature; and until God took our nature and became man, that way did not exist. God-made-man could love us with human love—and this, though a lesser thing than divine love, can be very comforting to our weakness. Nowhere in the Old Testament did it occur to anyone to God God what they were to call God-made-man, “the friend of sinners.” The Jews knew that God had spoken to and done great things for mankind, but He had not been man.
The moral for us is simple: in our approach to God we are helped enormously by seeing Him in our nature; and for the mind, this means a continual study of Him whereby the Apostles’ experience of Christ becomes our own personal experience, their intimacy becomes our intimacy. We cannot always analyze intimacy; but there is no mistaking it: we know the person quite differently. You do not learn intimacy, or reap the fruit of someone else’s. You grow into it. In the Gospels, one really can grow into this intimacy with Our Lord, precisely because the evangelists do not obtrude their own personalities. Anyhow, know Him we must. There is no other way to full knowledge of God; Christ has said so. In other words, we have to vivify all that hard thinking about the Infinite by the closest companionship with our Lord Jesus Christ. By both, the mind grows toward the knowledge of God which is its health (Sheed 82-83).
God is Love. Deo gratias.