God; a mystery yet knowable

His disciples said, "Ah, now you are speaking plainly, not in any figure!" (John 16:29)

We speak of the mystery of God and the Holy Trinity, but what does this mean? Didn't Jesus reveal all that we need to know? Hasn't the Holy Spirit been sent to lead us "into all truth"(John 16:13)?
Like most matters pertaining to Catholicism, the answer is both/and, not either/or.

The Holy Spirit has been sent to us to lead us and guide us, but also some things can't be fully understood by our finite human minds. We can believe that God is three persons, who are consubstantial with each other, and eternally coexistent as One God; but we can never fully understand that, and that's why the trinity is, and will always be, a mystery. CCC 261: "The mystery of the Most Holy Trinity is the central mystery of the Christian faith and of Christian life. God alone can make it known to us by revealing himself as Father, Son and Holy Spirit."

This takes us back to John 16:29, the apostles claim to understand perfectly what Christ is saying to them, but He seems unconvinced. He asks, "Do you now believe," He then goes on to tell them that they will be scattered and sure enough, that very night He is left alone after His arrest in the garden. If they truly had the full knowledge and understanding that they claimed to have, they wouldn't have been so easily shaken and scattered.

Had they really understood all that Jesus was saying then they would've understood the fact that no man had power over the Son of God, and nothing happened that He didn't allow to happen. As Jesus said to Pilate, "You would have no power over me unless it had been given you from above" (John 19:11a)

It wasn't until they were strengthened by the Holy Spirit, on Pentecost, that they began being bold, even though 7 weeks earlier they had claimed to understand. It's the same with us today; how many times have we said we understand when really we don't? How many times have we said to God, "ok I get it"?

Some matters can never be completely comprehended in this life, as I set out in the example of the trinity. There are many mysteries in our faith, some more difficult to comprehend than others, but this doesn't mean we throw our hands into the air and give up.

Ask yourself this question; do I now have more understanding and have more knowledge of the Christian life and of God than I did last year?
If the answer is yes, then good for you and keep moving forward, asking yourself that same question even on a monthly basis. If the answer is no, then now is a great time to start.

We don't all have to be philosophers or theologians, but if our understanding and knowledge of faith isn't any more advanced than it was when we first believed; then what does this say about our commitment? We are all called to share our faith and there is no shame in not knowing the answer to some questions, but it would be a bad witness to others if you said "I don't know, I haven't studied anything about my faith since Sunday school."

“We are talking about God. What wonder is it that you do not understand? If you do understand, then it is not God.” - St Augustine

In the end God is a mystery, that can never be fully comprehended; but we can know Him, we can have a relationship with Him. He isn't just someone we read and study about, like a long dead president or an influential thinker, He is a living God that calls us to live in a filial relationship with Him. It's this relationship that gives us life and is strengthened by the sacraments.