I’m wondering if your question is being asked in the face of possibilities, or whether you are in a relationship that has prompted this question. I am hoping the former, because otherwise, I’m sorry, this answer may lead to much emotional unrest.
The bible teaches us that, as Christians, we have freedom in Christ (Galatians 5:1). This freedom, that Jesus so dearly bought for us, is freedom from slavery to sin and death. We are now free to live as God, in his infinite wisdom, intends. We need to live as children of God (Philipians 2:15). We are called to put off our old selves (Ephesians 4:22); to love God with all our heart, soul, mind and strength and to love our neighbours as ourselves (Mark 12:30); to be living sacrifices, not conforming to this world, discerning what is the will of God (Romans 12:1-2); to walk in the light (Ephesians 5:8, 1John 1:5-7).
Someone who holds to agnosticism may be a “polite to possibilities” person, with no firm beliefs about God, or may think you can’t know about God. It is generally a position held where people feel the God question is too hard to answer, so they avoid it, living their lives with whatever standards they think best.
Marriage is intended to be a relationship of oneness between two people (Genesis 2:23,24) - two people who are intimate companions, complementing each other and encouraging each other as they work together toward the same goal. When a Christian marries an agnostic, or anyone of non-Christian belief for that matter, this unity of purpose and direction is not possible. That which is fundamental and central to the Christian person’s life is not fundamental and central to the non-Christian’s life. It is being unequally yoked (2 Corinthians 6:14) and can even be the cause for the Christian to fall away (2 Peter 3:17). So many compromises would have to be made in all priorities, like how you spend your time, money and energy. Raising children would be especially difficult because whose world view would you choose and would the other person support it?
For a couple to be growing in the same direction, they need to have their foundations the same. For a marriage to work as God intends, both husband and wife need to have their foundations resting firmly on Christ. Perhaps the most clear directive (though speaking to widows) is in 1 Corinthians 7:39 - marry “only in the Lord”.
I am hoping you will be resting in God’s word and in His love, trusting that His way is what is best for you. Proverbs 3:6 says “in all your ways acknowledge Him and He will make straight your paths”. I hope this has been helpful to you and I pray that God will help you see the straight path before you.