We can, because God the Father is real, Jesus the Son is real, and the Holy Spirit is real. If you are closed to God, then of course, it'd be impossible to know him. But if you are open completely and desire to know him well, he will let you know of his existence and presence. So it all depends upon you. Seeking union with God is the best way of knowing him intimately. In union, the Lord lets you feel and experience him in your heart, and anything the heart receives becomes part of you. Once you know God, you know him for ever.
A note. There is the so-called dark night of the soul some saints experience. During such a period, you no longer sense the presence of God ~ it's as if God has withdrawn altogether. This does not mean that you no longer know the Lord. To illustrate, you can have an intimate human friend with whom you are no longer able to communicate, but of course, you still know him well. Anyway, beginning seekers of God will not experience dark night of the soul and may never experience it. God grants it only to advanced souls for specific reasons.