For all the promises of God in him are yea, and in him Amen

2 Corinthians 1:20
For all the promises of God in him are yea, and in him Amen, unto the glory of God by us. (KJV)

The promises of God made to us are dependent on His ability and trustworthiness. On both counts, we can be sure that God is perfect. Hence there is no reason to doubt God's promises. The reason we may doubt is because the timing is slower than we think, and that we don't fully trust God enough because we don't know Him well enough,