When Jesus said; ‘if you have faith like a mustard seed’, it is before anyone was born again. But, after we are born again, the Holy Spirit gives to each one of us, a measure of faith – Romans 12:3
By the grace of God, each one of us, receives a different level of faith, the moment we are born again.
I believe, that we do not all receive the same amount of faith from God, because, God has a different plan and purpose for each of us. Some of us are called by God, to do greater things for him, than others, and so, they need a greater measure of his gift of faith.
For example a Christian who is called by God, to be a healing Evangelist, will need the faith to pray and see people healed by the gift of healing, which God has given them. So they will need to have a greater measure of faith, then other Christians.
All Christians can pray for healing and can flow in the gifts of the Holy Spirit, but some are called by God for a specific ministry, and that is different.
When we become born again, the Holy Spirit dwells in us permanently, and he gives us faith. Even one of the gifts of the Holy Spirit is faith. He gives faith to Christians.