The Catholic Encyclopedia:
"For nowhere in the Old Testament do we find any clear indication of a Third Person. Mention is often made of the Spirit of the Lord, but there is nothing to show that the Spirit was viewed as distinct from Yahweh Himself. The term is always employed to signify God considered in His working, whether in the universe or in the soul of man."
The New Catholic Encyclopedia:
"The formulation ‘one God in three persons’ was not solidly established, certainly not fully assimilated into Christian life and its profession of faith, prior to the end of the 4th century. But it is precisely this formula that has first claim to the title the Trinitarian dogma. Among the Apostolic Fathers, there had been nothing even remotely approaching such a mentality or perspective."
Alexei