I would recommend the following scripture for a couple building their foundation on Christ:

Matthew 7:24-27 - "Everyone then who hears these words of mine and does them will be like a wise man who built his house on the rock. And the rain fell, and the floods came, and the winds blew and beat on that house, but it did not fall, because it had been founded on the rock. And everyone who hears these words of mine and does not do them will be like a foolish man who built his house on the sand. And the rain fell, and the floods came, and the winds blew and beat against that house, and it fell, and great was the fall of it."

This scripture emphasizes the importance of not only hearing the teachings of Christ, but also putting them into practice. By building their relationship on a strong foundation of living out the principles and teachings of Jesus, the couple can withstand the challenges and trials that may come their way.